Female Swiss albino mice were injected intraperitoneally with poly I:C1 microgram/g body weight. The animals were killed 2, 12, and 24 hr after the injection of poly I:C. Half an hour before the death of animals they were reinjected intraperitoneally with 3H-thymidine. Fragments of ileum were excised. The quantification of autoradiographs included the counting of labeled cells, mitotic cells as well as the recording of cell position along the ileal crypts. The spatial distribution of both the mitotic and DNA-synthesizing cells of colonic crypts were affected by poly I:C at neither of time intervals between the injection and the death of animals. On the other hand, the parameters of cellular proliferation, i.e. mitotic and labeling indices, decreased significantly 12 and 24 hr after the injection of poly I:C, being unaffected 2 hr after the injection.