A rapid preincubated, charcoal-dextran radio-immunoassay procedure was applied to measurements of immunoreactive human chorionic gonadotropin (IR-HCG) during normal pregnancy and the results compared with those of biologic assays. Analysis of several available HCG samples of varying degrees of refinement indicates the need for an international reference preparation for radio-immunoassay. The characteristic pattern of serum IR-HCG during normal pregnancy is similar to that obtained by bioassay during the first trimester but differs significantly later in pregnancy, showing a gradual secondary rise to term.