In 190 pregnancies undergoing cordocentesis for prenatal diagnosis (n = 174) or elective caesarean section (n = 16), fetal peripheral blood B lymphocyte subpopulations were measured using a fluorescence-activated cell sorter (FACScan). The total number of B lymphocytes and polyreactive CD5+ B cells increased exponentially with gestation from respective means of 0.33 x 10(9)/l and 0.25 x 10(9)/l at 17 weeks to a plateau of 0.66 x 10(9)/l and 0.54 x 10(9) at 36 weeks, remaining at that level thereafter. The number of mature CD10- and active CD23+ B lymphocytes increased linearly from a mean of 0.07 x 10(9)/l and 0.11 x 10(9)/l at 17 weeks to 0.24 x 10(9)/l and 0.37 x 10(9)/l, respectively, at 40 weeks. As expected, all B lymphocytes expressed the HLA-DR antigen from as early as 16 weeks gestation. These alterations in specific B lymphocyte subpopulations reflect the pattern of maturation and development of the fetal humoral immune system.