The effects of dapsone (diaminodiphenyl sulfone) on the polymorphonuclear leukocyte (PMNL) function and on the elicitation of the passive Arthus reaction were investigated in rabbits and guinea-pigs. NBT-reduction was markedly enhanced in PMNL of animal receiving 5 mg dapsone/kg for 3 days, whereas chemotaxis and phagocytosis were not influenced. Passive Arthus reaction was clearly suppressed by the same treatment regimen. 2 mg/kg of the drug had no effect on the parameters examined.