Alpha-fetoprotein (AFP) is a fetal glycoprotein. It has been ascribed a regulatory function of growth factor responses and immune functions. The concentrations of AFP and albumin (ALB) are highly variable in fetal serum and CSF and change with gestational age. The AFP index=[AFP(CSF)/AFP(SERUM)]/[ALB(CSF)/ALB(SERUM)] was determined in six normal fetuses at gestational age 17-23 weeks and found to be independent of gestational age and close to unity, mean 0.90+/-0.11 (S.D.). The ratio of CSF-serum concentrations of AFP and ALB both decreased significantly (p<0.05) with gestational age. The mean fraction of AFP being non-reactive with concanavalin A was 1.7% in serum and 1.9% in CSF, suggesting a common hepatic origin of AFP in both compartments. In conclusion, the concentration of AFP in CSF seems to be determined largely by the serum-CSF concentration gradient in normal fetuses. This finding, combined with the remarkable constancy of the AFP index compared to the highly variable absolute concentrations of AFP in both serum and CSF should make the AFP index the marker of choice when analyzing for intrathecal AFP synthesis during development and in pathological conditions.