Although many immunostimulants have been shown to increase the lytic activity of natural killer (NK) cells in the periphery, little is known about their effects on NK cells in the bone marrow, the primary site of NK production. In the experiments reported here, we tested OK-432, a pharmaceutical preparation of Streptococcus pyogenes, for its effects on both the primary production and lytic activity of NK cells in C57BL/6J mice. NK activity in bone marrow cells (BMC) and spleen cells (SC) was significantly increased following intravenous administration of OK-432, peaking on day 2 in BMC and on day 3 in SC. Concomitantly, there were marked changes in the cellularity in the two compartments. Bone marrow cellularity fell significantly on day 1 post-OK-432 and then gradually returned to normal, whereas spleen cellularity rose rapidly and remained elevated. As a consequence, the total NK activity (per femur or per spleen) was significantly increased at 48-96 h after administration of OK-432. The target specificity was unchanged. The phenotype of NK cells in BMC as determined by cytotoxic depletion was unchanged by OK-432, but splenic NK activity shifted to a 'less mature' phenotype, intermediate between that of normal BMC and SC. Cytokinetic studies using 3H-TdR revealed an increase in the production of NK cells in the bone marrow following administration of OK-432. Proliferating NK cells also appeared in the spleen. Whether these were recently produced NK cells from the bone marrow that still retained the ability to proliferate or mature NK cells that were stimulated into cell cycle cannot be determined from these experiments. These data are the first to directly demonstrate the modulation of the primary production of NK cells by an immunologically active drug.