By Jud Pyle, CFA, chief investment strategist for the Options News NetworkBaxter International ( BAX) shares have climbed steadily since last week's eruption of swine flu concerns, and the stock continues to rally today. The medical instrument supplier's stock closed in the $49-handle on Monday and Tuesday, up 3% after closing at $48 on Friday. Looking at the June 52.5 call options, we see they are trading at about $1 per contract after closing at $1.08 Tuesday night, or an eight-cent drop. Normally, when the underlying stock rises, we expect to see the derivative rise as well. Even after BAX announced plans to work on a swine flu vaccine over the weekend, at least one investor is taking the opportunity of this rally to sell call options. BAX stock is currently trading around $49.90, up about 20 cents, or 0.43%, from yesterday's close. More than 6,300 call options have traded so far today, while the current open interest in these calls is 366. So what can we say about this type of call activity? For one thing, the call-selling has driven down implied volatility for the June 2009 52.5 call options. Using the options chain on the OptionsHouse.com platform, we can see that today's options price of a dollar vs. the stock price of $49.90 has an implied volatility of 27.4. In addition, we can intuitively see that implied volatility is lower because call options are lower despite a higher stock price. This call-selling activity does not mean investors should run right out and sell their shares. It does demonstrate how some investors are selling call options to protect some of the gains from the past two days. Despite today's rally, BAX shares hit a 52-week low of $46.63 just two weeks ago on April 7. This call sale could be a bet by a long stock holder that the calls will be hard-pressed to rally more in the next month and a half. Jud Pyle is the chief investment strategist for Options News Network (www.ONN.tv) and the portfolio manager of TheStreet.com Options Alerts. Click here for a free trial for Options Alerts. Mr. Pyle writes regularly about options investing for TheStreet.com.