Weight gain, feed efficiency and proliferation response of splenic lymphocytes were studied in mice fed either a yogurt or a milk-based diet. The yogurt contained liveLactobacillus bulgaricusandStreptococcus thermophilus.The feeding trial lasted for 10 months. Proliferation response of lymphocytes was measured by the uptake of tritiated thymidine into the cells when stimulated with the mitogens phytohaemagglutinin (PHA), concanavalin A (Con A) andEscherichia colilipopolysaccharide (LPS). There was no significant difference between the average body weight of mice belonging to the two dietary groups. But yogurt diet showed a higher feed efficiency ratio when compared to the milk diet for the first 8 weeks of the study while the mice were growing. After the period of growth was over, yogurt-fed mice maintained a constant body weight like the milk-fed mice but on a lower energy intake. The proliferation of splenic lymphocytes, expressed as Stimulation Index, was significantly higher in response to the T-lymphocyte mitogens Con A and PHA. Results suggest that yogurt has better feed efficiency and its long term inclusion in diets primes the splenic T-lymphocytes for a higher mitogenic response.