We reported in a previous paper that the pattern of change in the lysozyme content of normal amniotic fluid during pregnancy resembles indices of fetal maturity such as L/S ratio, creatinine concentration and amylase activity. In order to clarify the origin of amniotic fluid lysozyme and to determine whether or not the amniotic fluid lysozyme concentration indicates the maturity of some specific fetal organ, we measured the lysozyme content of samples of materials considered to be possible sources of amniotic fluid lysozyme. These materials were amnion and--taken immediately after birth--saliva, urine and cord serum. Lysozyme content was 36.5 +/- 6.7 micrograms/ml in the saliva samples, 5.3 +/- 0.9 micrograms/ml in the urine samples, and 17.4 +/- 4.4 micrograms/ml in the cord serum samples. It is unclear, however, which material was the most important source of amniotic fluid lysozyme. The results suggested that homogenized amnion samples contained lysozyme, although the content was low, and that amnion tissue produced lysozyme in vivo. Lysozyme is an enzyme found in the lysozymes of cells. The results of this study provide evidence that amniotic fluid lysozyme originates from many sources.