We are just about through with the spooky month on Wall Street, and the third-quarter earnings are starting to wrap up, and one thing we are seeing here is an increase in overall insider activity. As we have discussed in the past, companies shut insider trading windows when earnings are released to keep the appearance of any impropriety totally out of the question.

We cover insider buying every week at 24/7 Wall St., and we like to remind readers that while insider buying is usually a very positive sign, it is not in of itself a reason to run out and buy a stock. Sometimes insiders and 10% owners have stock purchase plans set up at intervals to add to their holdings. That aside, it still remains a positive indicator.

Here are some of the companies that reported notable insider buying last week:

Biogen Inc. (NASDAQ: BIIB) had a huge purchase hit the tape this past week, one that may very well turn some heads in the biotech world. A director at the company bought a stunning 310,000 shares of the biotech giant at prices between $276.77 and $277.70 per share. The total for the purchase was an incredible $86 million. The stock is down almost $200 per share since highs that were posted last March. Shares were changing hands at $290.51 as trading on Friday concluded, so it appears to be a well-timed buy.

Pentair PLC (NYSE: PNR) hits our screens again as an institutional account was buying shares last week. Trian Fund Management snapped up a block of 265,000 shares of the stock at prices that ranged from $54.23 to $56.30 apiece. The total for the buy came to a solid $14.7 million. Pentair operates as a diversified industrial manufacturing company in the United States, Europe and elsewhere. The share price ended the week at $55.92.

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Tenet Healthcare Corp. (NYSE: THC) had a 10% owner of the company adding to a position this past week. Glenview Capital Management bought a half million shares of the stock at prices between $28.13 and $30.50 apiece. The total for the buy came to $14.9 million. Tenet Healthcare primarily operates acute care hospitals and related health care facilities in the United States. The stock closed trading on Friday at $31.37, so another well-timed purchase.

Bank of New York Mellon Corp. (NYSE: BK) saw a director at the powerful banking giant purchasing shares last week. That director picked up a total of 285,000 shares of the stock at $41.34 apiece. The total for the buy came to an impressive $11.8 million. This investment company provides financial products and services to institutions, corporations and high net worth individuals in the United States and internationally. The stock was at $41.65 as trading ended on Friday.

Greenhill & Co. Inc. (NYSE: GHL) had the man at the top buying shares this week. The CEO acquired a total of 202,510 shares of the stock at $24.89 per share. The total for the buy came to an even $5 million. Greenhill operates as an independent investment bank for corporations, partnerships, institutions and governments worldwide. It provides financial advisory services primarily related to mergers and acquisitions, restructurings, financings and capital raises. The stock closed trading on Friday at $25.82, so a very well-timed purchase here too.

These companies also reported insider buying this week: Arctic Cat Inc. (NASDAQ: ACAT), Higher One Holdings Inc. (NYSE: ONE), JAKKS Pacific Inc. (NASDAQ: JAKK), Opko Health Inc. (NYSE: OPK) and Unifi Inc. (NYSE: UFI).

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One thing is for sure, the slow volume insider trading that started with the big market sell-off in late August may stay in the rear-view mirror for a while. With windows opening for executives to trade, and the end of the calendar year only eight weeks away, look for insiders to be very active.

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