We studied the responses to phytohemagglutinin (PHA) and Staphylococcus aureus protein A (SpA), a B cell mitogen, and surface markers in the thymus, liver and spleen of 5 human fetuses ageing from 12 to 26 gestational wk and in cord blood lymhocytes of 9 newborns. Sheep rosette forming cells appeared in all 3 organs at 17 wk; mIgM+ cells were detected in liver and spleen at 24 and 17 wk respectively. SpA and PHA stimulated the cells of all the 5 liver and all the 3 spleen samples examined. Thymus cells responded to PHA as early as the 17th wk and showed a progressive increase in stimulation indexes. An unexpected observation was the response of the thymus cells of a 15-wk fetus to SpA; this finding is discussed in the text. Cord blood lymphocytes gave results comparable to those found in normal adults.