BRT and CBR were determined from studies of radiobilirubin kinetics in 14 patients undergoing splenectomy for hereditary spherocytosis. Studies were conducted both before and after the operation in order to assess the effects of the postsplenectomy fall in BRT on CBR. Splenectomy led to a doubling of the RBC-t1/2 from 13.0 +/- 1.7 (S.E.) days to 26.1 +/- 1.5 and a fall in BRT from 15.9 +/- 4.4 mg/kg/day to 4.3 +/- 0.9. In seven patients (group A) initially normal values for CBR were unaltered by surgery. In the remaining seven (group B), low preoperative values for CBR, suggestive of Gilbert's syndrome, uniformly improved after splenectomy, becoming normal in five patients. Nevertheless, family studies and reduced values for UDPGT activity supported the initial impression of concomitant Gilbert's syndrome in group B. These studies suggest the existence of a latent state for Gilbert's syndrome that, in some patients, may be unmasked by a hemolytic stress. As such, they have important implications for both pedigree analysis and incidence studies of this condition.