The effects of prednisolone azathioprine and 6-mercaptopurine on polymorphonuclear neutrophil chemotaxis, phagocytosis, and killing of Staphylococcus aureus and Candida albicans have been studied. In twenty patients with a functioning kidney graft, taking both azathioprine 2.5 mg/kg/day and prednisolone (mean dose 0.59 mg/kg/day; range 0.30-1.0 mg/kg/day), the polymorphonuclear function did not significantly differ from that in either twenty-two normal or eighteen uraemic controls. Addition of prednisolone, 1.2 X 10(-5) M, azathioprine, 2.1 X 10(-5) M, and 6-mercaptopurine, 2.1 X 10(-5) M, using each drug alone, to normal human polymorphonuclear cells in vitro did not significantly alter their function. It is concluded that prednisolone and azathioprine together in vivo and that both these drugs and 6-mercaptopurine singly in vitro have no significant deleterious effect on polymorphonuclear function and do not contribute, in this way, to the increased susceptibility of patients receiving these drugs to infection.