Immunomodulatory effects of a dahi (strained yogurt) containing probiotic Lactobacillus casei, designated "probiotic dahi," was evaluated in Swiss albino mice during 8 days of feeding and compared with control dahi-, nonfermented milk-, and no milk product diet-fed (control) groups. Lysosomal enzymes (beta-galactosidase and beta-glucuronidase) and phagocytic activities were estimated in peritoneal macrophages of animals fed with different experimental diets on days 2, 5, and 8. The oral administration of probiotic dahi increased (by 84%) beta-galactosidase activity in supernatant of cultured macrophages and achieved the highest values on day 2 and thereafter decreased up to day 8, at which no effect on beta-glucuronidase activities was observed; it was the same as the control group. Moreover, phagocytic activity also increased in probiotic dahi-fed mice after days 2 and 5 and then decreased after 8 days, but was still higher than in nonfermented milk and control dahi-fed groups. The counts of total lactobacilli significantly increased after feeding of probiotic dahi as compared to the other groups. However, no significant changes were observed in beta-galactosidase and beta-glucuronidase activities and phagocytic activity of peritoneal macrophages isolated from nonfermented milk- and normal diet-fed groups. The results of the present study reveal that nonspecific immune response markers were stimulated in mice by feeding of probiotic dahi containing probiotic L. casei instead of milk alone and that the effect was greater than in control dahi.