A study of human parvovirus B19 infection in 1,967 pregnant women of all races in Johannesburg revealed an overall prevalence of 24.9% for IgG antibodies and 3.3% for IgM antibodies. Of the 64 IgM-positive sera indicating active infection, 62 were resistant to urea denaturation. No differences in the prevalence of IgG antibodies between population groups were observed, but active infections, as demonstrated by IgM antibodies, were significantly more prevalent in black than in white, coloured or Asian mothers.