An account of plasma cholinesterase activity in samples of maternal and cord blood is presented. It is confirmed that plasma exchange markedly reduces the level of activity in maternal blood, and that the level is further reduced during the first 3-4 postnatal days. A particularly marked decrease was found in those cases in which spontaneous mid-trimester abortion occurred. The level of activity in maternal blood (excluding mothers subjected to plasma exchange) at the time of delivery, was higher than that in cord blood in 61% of cases. In 23% of cases the level of activity was appreciably (0.05 units) higher in cord blood, and two-thirds of these cord samples contained the E2+ electrophoretic variant of plasma cholinesterase. The mean levels of activity in maternal and cord blood of Rhesus negative patients were significantly lower than those among Rhesus positive patients but there was no such distinction between Rhesus positive and Rhesus negative males and nonpregnant females. We encountered an incidence of 1:228 abnormal phenotypes in a series of 1593 mothers who underwent Caesarean section under a technique of general anaesthesia which included a suxamethonium infusion. However, probably only two of the seven patients would definitely be sensitive when not pregnant.