Lymphocytes of Macaca mulatta and Macaca fascicularis were cultured using a microculture technic. The use of RPMI-1640 with fetal calf serum as a culture medium and an extra dose of phytohemagglutinin produced excellent results. Staining of the chromosomes by the Gemsa banding technic did not reveal any differences between the karyotypes of the Macaca species.