Titers of mycoplasmacidal antibody to the human genital mycoplasmas Ureaplasma urealyticum and Mycoplasma hominis were determined using genital isolates from pregnant patients as antigens and comparing these isolates with the 11 prototypic reference strains for U. urealyticum and the seven reference strains for M. hominis. Virtually all titers that were detected with use of the patient's own isolates were detected by the 11 reference strains of U. urealyticum and by the seven reference strains of M. hominis. Serologic surveys of pregnant women who harbored either or both mycoplasmas in vaginal cultures indicated that antibody to M. hominis was found more commonly than antibody to U. urealyticum. It was demonstrated that significant postpartum rises in titers of antibody to M. hominis were correlated with the presence of these mycoplasmas in genital cultures. Postpartum rises in titer of antibody were particularly likely to occur in women with low titers of mycoplasmacidal antibody in serum at the time of delivery. Approximately 88% of the women who were colonized with M. hominis showed significant changes in titer of antibody to M. hominis throughout an apparently normal pregnancy; only 40% of the women who were colonized with U. urealyticum showed such changes in titers to U. urealyticum. Statistical analysis showed that mean log titers of antibody to both mycoplasmas at the first prenatal visit were significantly associated with the number of pregnancies experienced by these women.