Antineutrophil antibody capable of opsonising normal neutrophils was detected in the sera of 16 neutropenic patients receiving various drugs. A single drug was involved in 10 cases, multiple medication in 6. 4 patients received semisynthetic penicillins, 8 some form of antimicrobial, and 2 antithyroid agents. Neutrophil counts rose, and antineutrophil antibody activity declined or disappeared, after drug therapy was stopped. Opsonisation related to the semisynthetic penicillins was independent of complement but dependent on drug concentration. Complement-dependent opsonic activity was demonstrated in 2 patients (1 receiving methimazole and 1 receiving multiple medication) and in the patient receiving methimazole this activity was shown to reside in the IgM serum-fraction. In another patient receiving multiple drugs, a complement-independent IgG antibody was demonstrated.