Mitogenic responses of peripheral blood lymphocytes in naturally occurring clinical puerperal metritis and mastitis were investigated. Glucose consumption index (GCI) values for phytohaemagglutinin (PHA), concanavalin A (Con A) and pokeweed mitogen (PWM) in the puerperal metritic cows and mastitic cows were significantly lower than those in the healthy cows. Suppression of lymphocyte blastogenesis was correlated to an increased concentration of serum ammonia in the puerperal metritic cows, and of α1_-acid glycoprotein (α₁AG) in the mastitic cows. Lymphocyte blastogenesis in the mastitic cows was also correlated to the serum concentration of vitamin E. These findings indicate that the puerperal metritic and mastitic cows are associated with impaired lymphocyte blastogenesis.