Abstract Sixty milk whey samples prepared from quarters of five cows with a history of mastitis were tested for their ability to inhibit DNA synthesis in mitogen-stimulated lymphocytes. The inhibitory activity was compared with milk SCC, electrical conductivity, pH, and the number of colony-forming bacteria in the milk. Milk whey contained factors that inhibited DNA synthesis in cultured lymphocytes. Inhibition of mitogen-induced DNA synthesis increased with the clinical severity of mastitis and with increased values of indirect indicators of mastitis. The increases in inhibition and electrical conductivity were delayed past the increases in SCC. Milk whey (10 µl) from quarters with clinical mastitis and from quarters with SCC >900,000 inhibited 96 to 100%, 84 to 100%, and 69 to 100% of DNA synthesis in 3-d cultures of lymphocytes stimulated with Concanavalin A, phytohemagglutinin P, and pokeweed mitogen, respectively. The numbers of colony-forming bacteria correlated least with the inhibitory activity.