Fermented milk products may protect against breast cancer by stimulating immunologic activity. Twenty- five women [24.0 ± 0.7 (SE) yr] were assigned randomly to two groups: control (n = 12) and yogurt treatment (n = 13). Controls refrained from yogurt products for three months, whereas the yogurt treatment group consumed two cups (454 g/day) of commercially produced yogurt for three consecutive months. Prior yogurt consumption did not exceed 4-6 cups/mo, and subjects consumed their usual diet during the study. Three-day diet records and fasting midluteal blood samples were obtained during subjects' first, second, and fourth menstrual cycles (baseline, Month 1, and Month 3, respectively). Macronutrient intakes differed between groups only for carbohydrate. Calcium intake increased for yogurt consumers during intervention. Lymphocyte proliferation induced by concanavalin A, phytohemagglutinin, and pokeweed mitogen, interleukin 2 production, and cytotoxic T lymphocyte-mediated cytotoxicity was assessed after baseline and Months 1 and 3 for both groups. No significant immune differences between the control and yogurt treatment group were observed for concanavalin A, phytohemagglutinin, pokeweed mitogen, interleukin-2, or cytotoxicity. In conclusion, three months of yogurt consumption did not enhance ex vivo cell-mediated immune function in young women.