The nicotinic acid test was performed in 5 patients with Dubin-Johnson syndrome, 5 with Rotor syndrome and 13 with Gilbert's syndrome. The increment in serum bilirubin concentration and bilirubin retention 5 hr later differed; the proportion of conjugated bilirubin in the increment of total bilirubin in Dubin-Johnson and Rotor syndromes was greater than in Gilbert's syndrome. These observations suggest that the nicotinic acid test reflects, in part, impaired biliary excretion of conjugated bilirubin. The results did not differentiate the two conjugated hyperbilirubinemias. The nicotinic acid test was also performed before and after corticosteroid treatment in four patients with Dubin-Johnson or Rotor syndrome. Although serum total and unconjugated bilirubin concentrations were reduced by corticosteroids, no significant change occurred in the parameters of the nicotinic acid test, suggesting that corticosteroids may enhance uptake of bilirubin without significantly altering biliary excretion of conjugated bilirubin.