Studies on the change of peripheral T and B lymphocytes and T cells bearing Fc receptors for IgG and IgM in pregnant women were performed by using rosette-formation tests. There was no significant difference in the proportion of T and B lymphocytes between pregnant and non-pregnant women. The percentage of T cells bearing Fc receptors for IgG in the T lymphocytes which are considered to have suppressive activity increased in the various stages of pregnancy and post-partum as compared with that in non-pregnant women. On the contrary, the percentage of T cells bearing Fc receptors for IgM in the T lymphocytes which have a helper function decreased in pregnant and post-partum women. The results of this investigation suggest that the depression of cell-mediated immunity during pregnancy depends on the qualitative change of T lymphocytes, i.e. increased suppressor and decreased helper T lymphocyte subpopulations.