In order to investigate the antibacterial activity of amniotic fluid, the growth curves of several bacterial species in human amniotic fluid were observed by plate counting methods. The concentration of the bacteriolytic enzyme lysozyme in amniotic fluid was measured, and the relationship between lysozyme and antibacterial activity was discussed. The following results were obtained: Amniotic fluid had an antibacterial effect on B. subtilis, Staph. aureus and E. coli. When amniotic fluid was heated at 100 degrees C for 5 minutes, its antibacterial properties were lost or weakened. The lysozyme concentration in amniotic fluid increased gradually with gestational age, rising to 19.1 +/- 8.3 mu/ml at term. When lysozyme was added to amniotic fluid that had lost its antibacterial properties through exposure to heat, the antibacterial effect on B. subtilis was recovered. The growth of Staph. aureus in amniotic fluid exposed to heat was inhibited by the concomitant addition of lysozyme and aminobenzyl penicillin.