Immunological relationships in pregnancy were investigated in a longitudinal study of twenty-two pregnant women, whose blood samples were taken before pregnancy, during pregnancy, at delivery, and 3--5 months after delivery. Blood samples were also taken from the fathers and the infants, both at birth and after 3--5 months. All the samples were frozen by means of a cryobiological freezing system, and when a whole longtiudinal series had been collected, the material was thawed and tested in a single seance. T and B lymphocytes were studied with rosette tests (E and HEAC rosettes). T and B lymphocytes were found not to change during the course of pregnancy. It is thus concluded that the mother's tolerance of a fetus with a dissimilar tissue type is not exercised via changes in the total count of T and B lymphocytes, although there may well be changes in their subpopulations, with the hypothesis that T-suppressor function increases and B-lymphocyte function decreases.