summary Antibacterial activity, tested with B. subtilis, was present in all 44 human amniotic fluids at between 36 and 42 weeks of pregnancy and was unrelated to gestational age within these limits, or to maternal age or parity. High‐molecular‐weight (β‐lysin) activity, present in all samples, was directly related to concentration of bound zinc, but not to that of free zinc. Low‐molecular‐weight antibacterial activity (<1000 Daltons) was only present in nine specimens, but sub‐threshold concentrations were demonstrable by concentrating the ultrafiltrates in more than half of the remaining specimens. Low‐molecular‐weight activity in the ultrafiltrates was directly related to the concentration of bound zinc in the original amniotic fluid. Neither high‐ nor low‐molecular‐weight antibacterial activities were associated with amniotic fluid phosphate levels.

Summary. Antibacterial activity, tested against Bacillus subtilis, was present in all 34 samples of human amniotic fluid collected between 37 and 41 weeks gestation. High‐molecular–weight (β‐lysin) antibacterial activity was reduced after treatment with ethylene diamine tetra–acetic acid (EDTA), but not after treatment with 1, 10‐phenanthroline or diphenylthiocarbazone. The activity was restored following incubation with magnesium, calcium, or zinc. Low‐molecular‐weight antibacterial activity (<1000 daltons) in acid–alcohol extracts was reduced after treatment with dithizone, but not after treatment with EDTA or 1, 10 phenanthroline. Activity was restored after treatment with manganese, cobalt or zinc. High‐molecular‐weight (β‐lysin) activity was therefore biochemically similar to serum p‐lysin but biochemically different from the low‐molecular‐weight antibacterial component of amniotic fluid.

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