Serum 5-nucleotidase enzyme activity and serum sialic acid concentrations were measured in 219 pregnant women to establish the incidence of cholestasis of pregnancy and the effect of rapid fetoplacental growth on serum sialic acid level. Serum 5-nucleotidase activity increases in diseases of the liver and obstruction of the biliary tract, whereas the level of serum sialic acid, a carbohydrate component of plasma membrane and cellular glycoproteins, rises in the presence of rapidly proliferating cells such as tumors. Serum 5-nucleotidase activity did not change between the first and the third trimesters of pregnancy. The elevation of serum 5-nucleotidase activity and of alkaline phosphatase in one case most likely indicated a cholestatic effect of pregnancy, and represented an incidence of 0.45% in the women studied. Among 201 cases, we found a graded and significant increase in the mean (+/- SD) serum sialic acid concentration with progressing pregnancy, from 1.63 +/- 0.3 mmol/L at the onset of pregnancy (five to eight weeks) to 2.06 +/- 0.49 at term; in the last stages of pregnancy, some of the values may overlap with those of cancer patients. We could not find any earlier report in the literature regarding the increase in serum sialic acid level in pregnancy.