Contents and relating biosynthetic enzyme activities of collagen in human placenta were studied, and its characteristics and possible role were discussed. Main results were as follows. Hydroxyproline contents of the normal placenta in the 1st, 2nd and 3rd trimester were 7.2 +/- 1.1, 14.6 +/- 4.4 and 23.4 +/- 4.8 micrograms/mg protein, respectively, and increased linearly as the pregnancy progressed (p less than 0.01, r = 0.8753). Among cases of abnormal pregnancy, one of three cases of intrauterine stillbirth and cases of light-for-date baby pregnancy with severe toxemia showed significantly higher hydroxyproline content than those of others. Prolyl hydroxylase activities of the normal placenta in the 1st, 2nd and 3rd trimester were 1048.2 +/- 174.0, 381.2 +/- 76.7 and 277.9 +/- 103.7 c.p.m/mg protein, respectively. Significantly higher activities were found in the 1st trimester than in the 2nd and 3rd trimesters (p less than 0.01). In abnormal pregnancy, the activities of the light-for-date baby with severe toxemia were significantly higher levels than those of others (p less than 0.05). These results may suggest that the collagen in the human placenta is not only concerned with developmental construction of the placenta, but also associated with complicated metabolic function of the placenta as well.