Amniotic fluid samples (140 from 111 patients) were obtained by transabdominal or transvaginal amniocentesis and examined for “fat cell” counts and creatinine concentrations. Infant weights at birth were used as the measure of maturity. Fetal maturity was correctly indicated by 94% of the fat cell counts of 20% and greater, and by 97% of the creatinine concentrations of 1.5 mg/100 ml or more. Fat cell counts lower than 20% and creatinine concentrations lower than 1.50 mg/100 ml were less accurate in indicating a premature baby.