Venous blood samples were collected prospectively in ten menstrual cycles where conception occurred, and for several weeks thereafter, and the circulating concentrations of pregnancy specific beta 1 glycoprotein (SP1) and human chorionic gonadotrophin (HCG) determined using specific radioimmunoassays. The data were converted to logarithms and pooled and pooled using the day of the LH peak as zero time. The concentration profiles so obtained were remarkable similar and showed a rapid exponential increase in concentration during the very early stage of pregnancy. The doubling times were 2.4 days and 2.3 days for SP1 and HCG respectively and both proteins were initially secreted in approximately equivalent concentrations. However, some 30 days following the LH peak, the concentration profiles began to diverge, with SP1 levels continuing to rise whilst HCG concentrations showed the characteristic plateau at 8 and 10 weeks of gestation.