The distribution of cloxacillin and flucloxacillin in whole blood from seven newborn infants and their mothers was determined in vitro by equilibrium dialysis at 37 degrees C. Seven healthy, non-pregnant women of reproductive age served as controls. The distribution of the penicillins to erythrocytes was the same in the infants as in the adults. It was significantly lower in the presence of plasma albumin than when plasma was replaced by isotonic phosphate buffer. The plasma protein binding of cloxacillin and flucloxacillin in 22 infants was significantly lower than in the controls, but was slightly higher than in the mothers. A significant correlation between binding of cloxacillin and flucloxacillin in the same individual suggested that the two drugs were bound to similar sites. During the first postnatal week binding in infant plasma decreased. This change was correlated with an increase in the bilirubin levels. In the mothers, the binding increased during the first week after delivery. On the basis of the distribution data, maternal to fetal plasma and whole blood concentration ratios at equilibrium were calculated. These ratios were lower for flucloxacillin (medians 0.770 and 0.821, respectively) than for cloxacillin 0.996 and 1.094). Accordingly, at equilibrium somewhat higher levels of flucloxacillin should appear on the fetal than on the maternal side, whereas the concentrations of cloxacillin would be expected to be approximately the same.