The effect of the capsular polysaccharide of Klebsiella pneumoniae type 1 Kasuya strain (CPS-K) on the formation of macrophage colonies in cultures of mouse spleen cells was investigated by the liquid culture technique during an incubation period of 7-8 days. CPS-K markedly inhibited further generation of macrophage colonies when added at any time after the beginning of culture, whereas it showed no destructive effect on macrophage colonies which were already formed before its addition. When CPS-K was present throughout the incubation period, such a low concentration as 0.05 micrograms/ml significantly inhibited colony formation, and the intensity of its inhibitory effect depended on its dose in the range of 0.005-50 micrograms/ml. The inhibitory effect persisted even if CPS-K was washed out after spleen cells were kept in contact with 20 micrograms of CPS-K per ml at 37 C for 6 hr. It was found that the inhibitory effect of CPS-K on colony formation was not mediated through its action on T cells, B cells or macrophages, and that it was not due to the generation of suppressor cells capable of inhibiting colony formation. It is concluded therefore that CPS-K directly inhibits the proliferation of macrophage colony-forming cells. The active substance responsible for the inhibitory effect of CPS-K on colony formation is the neutral polysaccharide fraction of CPS-K.