The addition of maternal serum intact hCG (MShCG) to routine maternal serum alpha-fetoprotein screening for Down's syndrome is expected to yield a detection efficiency around 60% for an amniocentesis rate approximating 5%. We compared the detection rate using intact MShCG and free-beta hCG in 480 normal pregnancies and 48 with chromosome defects (Down's syndrome 31, other chromosome defects 17). No significant difference in detection efficiency was determined. However, since the false-positive rate with free-beta hCG was almost twice that found with intact hCG, and the detection rate for other chromosome defects was more than double, the intact MShCG assay is currently preferred. Free-beta hCG earlier in gestation may, however, ultimately prove superior in maternal screening for chromosome defects.