Serum concentrations of different bile acids (BA) were determined by radioimmunoassay in 56 human fetuses and mothers. Serum was obtained immediately after legal abortion, performed between the 14th and the 21st wk of gestation. Conjugated cholic (CCA) and chenodeoxycholic acid (CCDCA) concentrations were determined in 33 cases, conjugated lithocholic (CLCA) and deoxycholic acid (CDCA) in 20, and sulfolithocholyglycine (SLCG) in 15. In fetal blood, mean concentrations of CCA (0.80 +/- 0.40 mumol/liter), CCDCA (4.50 +/- 2.70 mumol/liter), and CLCA (1.70 +/- 1.04 mumol/liter) were significantly higher than those in the mother (CCA 0.34 +/- 0.17 mumol/liter; CCDCA 0.79 +/- 0.34 mumol/liter; CLCA: 0.70 +/- 0.30 mumol/liter; p less than 0.001); fetal serum levels of CDCA (0.46 +/- 0.32 mumol/liter) and SLCG (0.15 +/- 0.09 mumol/liter) were lower than in the mothers (CDCA 1.20 +/- 0.80 mumol/liter, p less than 0.001; SLCG 0.40 +/- 0.30 mumol/liter, p less than 0.01). There was no correlation between levels of BA and gestational age. Serum total protein and albumin concentrations were both reduced in 10 fetuses as compared with the mothers. These data support the concept of a state of physiologic cholestasis during development and suggest that placental transfer of primary BA occurs mostly in the fetal to maternal direction. This transfer could be facilitated by the reduced fetal plasma albumin concentration, since BA in free solution diffuse more easily through the placenta. There is evidence of lithocholic acid synthesis in the fetal liver, while deoxycholic acid appears to be mostly of maternal origin. Finally, sulfation of BA is poorly developed at this age of gestation.