Optimal conditions for stimulation of bovine lymphocytes by Phytohaemagglutin (PHA) were established in a microtitre assay. Lymphocytes from efferent duct lymph responded vigorously, with specific incorporation of 100,000 to 200,000 cpm/well, providing the conditions of cell culture are carefully controlled. Peripheral blood lymphocytes responded over a wider range of cultural conditions but optimal specific incorporation was usually only 40-100,000 cpm/well. © 1979.