Fourteen renal transplant and one heart transplant mothers receiving cyclosporin (mean dosage: 273 +/- 19 mg/day) underwent a Caesarean section at a mean gestational age of 34.1 +/- 1.9 weeks. Circulating cyclosporin was assayed by HPLC. The mean blood levels in the mothers before the Caesarean section were 210 +/- 16 ng/ml, in cord blood 62 +/- 16 ng/ml (14 infants) and in the peripheral blood within 6 h of birth 31 +/- 12 ng/ml (15 infants); there was no correlation between maternal and cord levels, nor between peripheral blood levels at 2 h and cord blood levels. Cyclosporin levels were undetectable at day 3 in 12 infants, but low levels were found in 1 infant up to day 12. There was no toxic effect on the fetus or neonate.