Human parvovirus B19 (B19) crosses the placenta causing fetal death. We used an indirect capture enzyme immunoassay to measure IgG to B19 in sera of 845 subjects from 2 groups. The first group included 405 women (mean age, 30 years) composed of 85 pediatric nurses and 130 other female hospital employees, 122 women employed caring for preschool children and 68 mothers of preschool children enrolled in day care. Twenty-eight percent of all these women were seropositive. Seropositivity was unrelated to occupational group. Four of 235 women observed between 1983 and 1987 for a mean of 435 days/woman acquired B19 infections (an annual seroconversion rate of 1.5%). We investigated intrafamilial associations of B19 infection in a second group of 440 subjects from 111 families. Seropositivity of parents was not associated with seropositivity of their children. Seropositivity of one spouse was not associated with seropositivity of the cospouse. However, of 47 seropositive older siblings, 32 (68%) of their younger siblings were seropositive, compared to 20 (18%) seropositive younger siblings of 112 seronegative older siblings (P less than 0.001). B19 infections increased with age from 19% for those younger than 10 years to 67% for those older than 49 years. For all ages females had a higher rate (51%) of B19 infection than males (38%). These data suggest that children may be more susceptible to B19 than adults and B19 infections occur infrequently among women younger than 40 years of age. However, during local outbreaks the B19 infection rate for susceptible pregnant women remains unknown.