Twenty-eight mothers smoking 10--20 cigarettes daily during pregnancy had significantly higher serum thiocyanate concentrations at delivery compared in 25 non-smoking controls. The thiocyanate levels were positively correlated to cigarette consumption and inversely correlated to the birth weights of the infants. A highly significant correlation existed between serum thiocyanate levels of the mother and umbilical cord serum thiocyanate levels, reflecting a nearly complete equilibration. The thiocyanate concentrations in human milk on the 4th day after delivery were considerably lower than the serum concentrations, and no correlation existed between serum and milk concentrations. The infants of smoking mothers had significantly decreased weight and length at birth compared in infants of non-smokers. Birth weights were 3 344 +/- 434 g and 3 620 +/- 504 g respectively (p less than 0.05), and lengths 49.8 +/- 1.7 cm and 51 +/- 1.6 cm respectively (p less than 0.05). No differences were found between smokers and non-smokers in placental and umbilical cord histology, and umbilical cord artery medial area. It is concluded that serum thiocyanate concentration in smokers may be used as an objective measure for smoke exposure, and that maternal cigarette smoking acts as an exogenous factor which interferes with intrauterine development of the fetus in a dose related way.