Health stocks closed the week on a positive note as analysts adjusted their stance on a few individual stocks.

The Nasdaq and Amex biotech indices were up 1.8% and 1.5% Friday while the Amex pharmaceutical index was up 0.9% at 286.68.

One stock not registering a gain, Maxygen ( MAXY) slid $1.36, or 28%, to $3.49 after disclosing in a Securities and Exchange Comission filing that the U.S. Patent and Trademark Office granted a patent to Amgen ( AMGN) on June 3 that Amgen may contend covers Maxygen's MAXY-G34 product candidate for chemotherapy-induced neutropenia, a low number of the bacteria-destroying white blood cells that defend against infections.

Maxygen said its current activities are exempt from patent infringement liability, but if and when it commercializes MAXY-G34, Amgen might begin an infringement action. In that case, Maxygen believes it would have a viable defense. But, the company warned, "If we are unable to show that Amgen's patent is invalid and we are unable to obtain a license from Amgen for the use of their intellectual property, this may materially impact our ability to manufacture and sell MAXY-G34."

Amgen shares traded up 75 cents, or 1.7%, at $43.97. The stock is a component of the aforementioned Amex biotech index.

Shares of fellow component Gilead ( GILD) were up $1.41, or 2.7%, at $54.14. The company said Friday that its inhaled drug for cystic fibrosis continued to meet safety goals in an interim analysis of an ongoing, open-label study dubbed AIR-CF3. Data were presented at the Annual European Cystic Fibrosis Conference, which runs June 11-14 in Prague. The Food and Drug Administration is scheduled to make its approval decision on Sept. 16.

Elsewhere, ImClone ( IMCL) shares fell 60 cents, or 1.5%, to $38.50. Wall Street has a variety of opinions on the potential of ImClone's cancer drug Erbitux. On Thursday, Goldman Sachs analyst May-Kin Ho reaffirmed a buy rating with a $50 price target, while on Friday bearish Morgan Stanley analyst Steven Harr, who has an underweight rating, dropped his price target to $35 from $38.

In other analyst actions, First Analysis Securities upgraded Invitrogen ( IVGN) to overweight from equal weight, a day after the company announced a $6.7 billion buyout of Applera's Applied Biosystems ( ABI). Invitrogen shares fell 48 cents, or 1.2%, to $38.25 Friday.

Credit Suisse downgraded PDL BioPharma ( PDLI) to neutral from outperform. Its shares traded down 37 cents, or 3.3%, to $10.75.

Also, UBS downgraded Accuray ( ARAY) to sell from neutral -- shares were off by 9 cents, or 1%, at $8.89 on slightly lighter-than-average volume.