Total serum IgM levels were studied in 84 mothers of infants with group-B streptococcal (GBS) septicemia/meningitis and compared to IgM concentrations in 91 parturients who were urogenital carriers of GBS but nevertheless gave birth to healthy infants. In all, 22 (27%) in the study group showed IgM levels above the arbitrarily selected limit of 2.40 g/l, in contrast to 12 (13%) of 91 controls (p = 0.02). Among the study group members whose infants were infected with GBS type III, 8 of 34 (24%) were high in serum IgM, compared to only 2 of 34 (6%) of the corresponding controls (p = 0.04). The total serum IgG levels did not differ between the two groups.