In this review the mechanisms whereby amniotic fluid serves to protect the fetus from microbial disease have considered. It appears from the data reviewed that the principal mode of antibacterial action of amniotic fluid is bacteriostasis. Thus, the host is able to cope with a small number of organisms introduced into the amniotic cavity; however when the amniotic fluid is grossly contaminated the host resistance capacity of the amniotic fluid may be overwhelmed. This may be understood best by the quantitative description of disease theroized by Theobald Smith (94). Smith suggested that disease was a function of the number of organisms with which a host is initially infected multiplied by the virulence of the organism. The effects of the number and virulence of the organism in producing disease is lessened by the host's resistance capacity. This concept of disease is summarized by the equation: Disease equals (number) (virulence)/resistance. Although these parameters do not possess numeric values, it is possible to see at least conceptually their interplay with respect to intrauterine infection. For example, the number of organisms reaching the amniotic fluid may be increased by various modes, namely maternal viremia or bacteremia; premature rupture of the fetal membranes, antenatal vaginal examination and possibly intrauterine fetal monitoring. While these circumstances may result in increased rate of infection, some reports conversely indicate that minimal bacterial contamination in the amniotic fluid is not an extraordinary occurrence and may not result in any maternal or fetal complication (73,74). The intrinsic host resistance capacity of the amniotic fluid likewise represents an important part of the Smith equation for disease. We have found that amniotic fluids may vary in antibacterial efficacy from almost no inhibitory activity to profound bactericidal activity (90). Obviously, the likelihood of the production of disease by an equivalent inoculum of a particular organism would be quite different depending upon the intrinsic inhibitory capacity of the amniotic fluid. The measurement of the inhibitory capacity of amniotic fluid holds some promise for enabling a physician to determine which patients may be at special risk of intrauterine infection.