SUMMARYFrom bovine mammary secretion during the dry period, the total number of cells was between 1·2 and 5·9 Ǻ 106/ml. A mean of 35% of these cells were classified as lymphocytes and ∼ 85% of them could be isolated by the Ficoll-isopac method. Centrifugation separated 6% of the cells into the fat; 5% of them were lymphocytes. About 47% of the lymphocytes boundHelix pomatiaagglutinin, a T-cells marker, while the proportion of Ig-bearing cells was ∼ 28%. The mammary lymphocytes were stimulated by the lectins phytohaemagglutinin, pokeweed mitogen, concanavalin A and by lipopolysaccharide fromSalmonella typhimurium. The stimulation indices of mammary lymphocytes were generally lower than those for peripheral blood lymphocytes from the same animals. The background values, i.e. counts/min of lymphocytes incubated without mitogen, were often higher for lymphocytes isolated from mammary secretion than from blood.