It was previously demonstrated, by means of fixed tissue sections, that cell-free spleen extracts are capable of accelerating the general radiation recovery of spleen and bone marrow. For quantitation of various bone marrow cell types, further work using smear preparations rather than sections was conducted. The results showed accelerated recovery of bone marrow cells in guinea pigs treated with heterologous (canine) spleen extract following exposure to a single, whole-body dose of 650 r Co/sup 60/ gamma radiation (LD/sub 79/20/). A purified fraction of dog spleen extract which reduced radiation mortality, exhibited a similar trend. Erythrocytes, reticulocytes, granulocytes, megakaryocytes, and mitotic activity all appeared to be influenced in the same manner. The 7th postirradiation day results showed that spleen extract treatment does not protect bone marrow cells against the initial impact of irradiation. This indicates that the reduction in radiation mortality following spleen extract treatment is related to a recovery rather than a protective mechanism. Accelerated recovery in the spleen-treated animals is most evident on the 14th postirradiation day. Erythrocytic regeneration begins at this time in the spleen- extract-treated animals, as evidenced by increased marrow reticulocytes. This finding substantiates the previous observation of intense erythropoietic activity in the splenic red pulpmore » of extract-treated animals at this time. These observations support the contention of an induction of stem cell activity, and possibly of accelerated maturation of various cellular elements of the bone marrow, as a mechanism by which spleen extracts decrease radiation mortality. (BBB)« less