Aqueous extracts prepared from porcine kidneys (PKE) were found to promote colony formation in agar cultures of mouse bone marrow cells stimulated by colony stimulating factor (CSF) and to exhibit no detectable activity of CSF. The extract (PKE) enhanced colony formation about twice in comparison with control. On the cultures of the marrow cells from 5-FU injected mice with PKE, colony enhancement ratio increased to 3 and 1.5 times in comparison with that of control and that of 5-FU untreated mice, respectively.Futhermore, preincubation of bone marrow cells with PKE for 2 or 3 d caused an increase in the number of colony forming units in culture (CFU-C) by a factor of 1.5 of its initial number, whereas preincubation without PKE caused a considerable decrease of CFU-C number.These results suggest that the colony promoting activity of PKE influenced colony growth by enhancing the target cells of CSF. And there is a possibility that the colony promoting activity of PKE is a physiological factor in maintaining the number of CSF-responsive cells by proliferating and differentiating pre-CFU-C.It is found that there is a similarity between the colony promoting activity of PKE and the other sources in regard to heat sensitivity, trypsin sensitivity and some properties of their target cells.