115 pregnant girls aged 13-17 years were investigated during the third trimester for endocervical infection with six sexually transmissible microorganisms. Specimens from 21 patients destroyed the tissue cell monolayers for propagation of Chlamydia trachomatis, but 11 were no longer toxic when recultured after freezing or with additional antimicrobial agents; Trichomonas vaginalis was present in 76% of the toxic specimens. C trachomatis was recovered from 37% of 105 specimens. T vaginalis was recovered from 34% of the 115 subjects, candida from 38%, Mycoplasma hominis from 70%, and Ureaplasma urealyticum from 90%. Neisseria gonorrhoeae was cultured from 1 of 12 girls infected earlier in pregnancy. T vaginalis infection, alone or with C trachomatis or candida, was associated with low gestational age and low birthweight. C trachomatis and candida infections alone had no effect on pregnancy outcome.