Lactating goats given an intramammary inoculation of 50 Mycoplasma putrefaciens in 1/2 of the udder had a high percentage of leukocytes in the milk within 8 days, without involvement of the other half. Larger doses of mycoplasma caused earlier inflammatory changes, but there were no differences in the final disease. In all instances, the viability count of the mycoplasma reached at least 10(8) organisms/ml before a high percentage of neutrophils were present in milk smears. The California mastitis test was 2+ to 3+ when 10(10) M putrefaciens/ml were cultured from the secretions. The main features of M putrefaciens infection in the udder are the lack of clinical signs and rapid growth of the organisms, followed by inflammatory cells in the milk and complete halt to lactation.