We have evaluated 33 patients with various bone marrow dyscrasias treated with horse antithymocyte globulin (ATG) (Upjohn) to determine the relationship of haematopoietic response to the occurrence of serum sickness. Patients received ATG intravenously over 10 or 28 d at a dose of 15 mg/kg/d. Total or partial haematological responses were noted in 12 of 33 patients. Twenty-eight patients developed clinical signs of serum sickness 6-14 d after the first infusion of ATG, while five patients did not. Twenty-five of these patients were evaluated by immunochemical assays for circulating immune complexes (C1q-binding assay) and 21 patients for serum complement (C3, C4 and CH50 assay) levels. There was a direct correlation between increases in immune complex levels, decreases in serum complement levels, and the development of the clinical signs and symptoms and serum sickness. Twenty-one of 28 patients who developed serum sickness failed to show haematological improvement. However, haematopoietic recovery occurred in all five patients who did not manifest serum sickness (P less than 0.05) and in four patients who failed to develop immune complexes. These findings indicate that the development of serum sickness is not required for a haematopoietic response with ATG and may indeed impair recovery.