In our experience Δ 450 mμ. OD measurements have shown a progressive decline during the latter half of gestation in pregnancies unaffected by fetal hemolytic disease. However, we have seen two Rh-negative patients in whom there was a rise in Δ 450 mμ OD between 24 and 27 weeks' gestation and who subsequently bore Rh-negative infants. In these two cases the increase between 24 and 27 weeks was from approximately 0.140 to about 0.200. Both infants were unaffected with fetal hemolytic disease. Therefore, we recommend caution in interpreting modest increases in Δ 450 mμ, OD during this period of gestation.