Development of splenic haemopoiesis and morphology of lymphocytes in the embryonic and neonatal mice were examined by light and electron microscopy. At 15 days of gestation, free mononuclear cells were scattered throughout the splenic anlage, and the spleen was prehaemopoietic. At 16 days of gestation, immature erythroid cells and small lymphocytes first appeared and then the spleen became predominantly erythropoietic. In the red pulp, lymphocytes constituted approximately 2% of haemopoietic cells during fetal life and 3.6% at 4 days after birth. Splenic lymphocytes in the embryo had a nucleus 3.5-4.5 microns in diameter, and the majority were small lymphocytes with sparse and dark cytoplasm. Small lymphocytes occasionally had an invagination of the inner nuclear membrane into the nucleoplasm, showing various sectional profiles; in ultrathin sections, the proportion of small lymphocytes having the nuclear membrane invagination comprised 14.3% of the small lymphocytes in the embryonic spleen.