We used a highly purified preparation of the naturally occurring core fragment of hCG beta (beta-core) and a new RIA for beta-core to investigate the concentrations and behavior of beta-core in serum and urine. We collected serum and 24-h urine specimens from healthy pregnant women during the first trimester of pregnancy. The concentrations of beta-core in serum were determined by analysis of fractions eluted from Sephadex G-100. Serum concentrations of beta-core immunoreactivity were very low (0.13-1.25 micrograms/L), while large amounts of beta-core were excreted in urine during pregnancy (as much as 4-5 mg/day). Interference with measurement by serum factors did not account for the low levels of beta-core immunoreactivity in pregnancy serum. Based on the known urinary clearance rate of beta-core in healthy nonpregnant subjects, we calculated that urinary clearance of serum beta-core accounts for only about 1% of the beta-core in pregnancy urine. We conclude that during pregnancy, the concentrations of beta-core in plasma are measurable, but extremely low, and that most of the beta-core in urine is derived by mechanisms other than urinary clearance of serum beta-core; most likely, these mechanisms involve nephrogenous production of beta-core from precursor molecules such as hCG and hCG beta.