Phytosterols play a major role in functional foods. Their ability to reduce serum cholesterol in humans has been widely proven and they are now being added to various dairy based products. The present study investigated the potential antimicrobial activity of phytosterols in milk and their effect on yogurt starter cultures. A commercial phytosterol (0.26% to 1.8%, w/v) preparation (CPP) consisting of ss-sitosterol, campesterol, sitostanol, and campestanol had no effect on the standard plate count (SPC) and psychrotroph population in pasteurized milk stored at 4 degrees C. In addition, a challenge study employing Pseudomonas spp. in milk at 4 to 7 degrees C confirmed that the CPP was not antimicrobial. However, the addition of a dispersible CPP consisting of 0.72% phytosterol containing 0.02% to 0.03% sodium stearoyl lactylate (SSL) did appear to affect the SPC and psychrotrophic bacteria in refrigerated milk. The dispersible preparations did not, however, inhibit the growth of Pseudomonas. An investigation into the antimicrobial activity of SSL revealed that it alone had no effect on the SPC in milk. The CPP had no effect on growth and acid development by Lactobacillus bulgaricus and Streptococcus thermophilus during yogurt production at 33 degrees C and storage at 4 degrees C for 30 d. This is seen as a beneficial feature since growth and acid development by these organisms are crucial for yogurt quality. Saccaromyces cerevisiae and Aspergillus ochraceous added to yogurt as typical contaminants also were not inhibited. While the CCP was somewhat antimicrobial when formulated with dispersing agents, it otherwise had no antimicrobial activity.