Samples from 61 amniotic fluids (33 discolored and 28 clear) collected by amniocentesis between 16 and 20 weeks gestation were cultured for microorganisms. Two of the discolored fluids were positive for Mycoplasma hominis, and two were positive for Ureaplasma urealyticum. Neither organism was isolated from samples of clear amniotic fluids. No other bacteria, viruses, or chlamydiae were isolated from fluids of any patient. The results prove that both M. hominis and U. urealyticum can infect the amnionic sac early in gestation without rupture of the fetal membranes. U. urealyticum was shown to cause a clinically silent, chronic (up to two months) intrauterine infection characterized by an intense inflammatory response. Complications in three of the four patients from whom mycoplasmas were isolated suggest, but do not prove, that mycoplasmal infection of amniotic fluid may have an adverse effect on outcome of pregnancy. Further studies are necessary to substantiate this implication and to determine the relationship between infection and discoloration of amniotic fluid. © 1983 American Sexually Transmitted Diseases Association.