Among 71 couples with histories of pregnancy wastage, 84.5% were colonized with Ureaplasma urealyticum and/or Mycoplasma hominis; whereas in couples with successful deliveries the incidence was 25.4%. The distribution of U. urealyticum and M. hominis was comparable in the fertile and infertile populations. Of women with positive cultures, 96% aborted, compared with an expected rate of 19% to 45%. Serologic studies revealed that, at delivery, 42.9% of infants of mothers with pregnancy losses had fourfold elevations in titers above the mothers' level compared with 15% of normal infants. Mothers with pregnancy wastage histories had elevated titers above their infants in 42.9% of cases compared with 10% of normal mothers. Thus, both mothers and fetuses had responded immunologically to the presence of U. urealyticum. When the mean antibody titers in the normal and pregnancy wastage groups were calculated for each ureaplasma serotype, the infants of mothers with pregnancy losses exhibited significantly elevated mean titers to serotypes 6 and 8, while the mothers had elevated mean titers to serotypes 4 and 8. These observations suggest that U. urealyticum causes infection in mothers and fetuses and that certain ureaplasma serotypes may be more pathogenic than others.