In order to study the activation of complement by soluble aggregates of human polyclonal serum IgA, lysis of sheep erythrocytes (E) coated with several IgA preparations was used as a model. A complement nonactivating monoclonal mouse IgG₁ against IgA was used to coat the cells. IgA, isolated from normal human serum, was aggregated by either N‐succinimidyl 3‐(2‐pyridyldithio)propionate (SPDP), glutaraldehyde, carbodiimide or heating. Depending on the size of the aggregates, and on the method of aggregation, E coated with aggregated IgA (Eγ₁.AIgA) could be lysed. The alternative pathway of complement appeared to mediate the lysis because the latter was observed in the presence of EGTA containing 5 mM Mg²⁺ (MgEGTA) and properdin (P) was deposited on the cells. Furthermore, no lysis was observed in C3‐deficient serum. In the absence of AIgA the cells were not lysed, and no P deposition was observed. In another set of experiments Eγ₁.AIgA were first reacted with purified C3, B, D and P for 30 min at 30°C, and subsequently in rat serum EDTA at 37°C. Lysis occurred when Eγ₁.AIgA were prepared using SPDP‐, glutaraldehyde‐ or carbodiimide‐AIgA. Incubation of 100 μg/ml SPDP‐AIgA with normal human serum for 30 min at 37°C in the presence or absence of MgEGTA also induced consumption of total complement. The other soluble AIgA preparations were less effective in activating complement. These results suggest that polymeric serum IgA is capable of activating the alternative pathway of complement.