The nitrophenyl-specific IgA myeloma protein secreted by the MOPC-315 mouse plasmacytoma was examined for its capacity to inhibit antibody-mediated hypersensitivity reactions in the mouse. Specific inhibition of both cutaneous anaphylactic and complement-dependent (Arthus) reactions was observed in sensitised mice when the eliciting antigen was mixed with IgA prior to injection or when the mice injected with plasmacytoma cells and possessed detectable amounts of MOPC-315 IgA in their serum at the time of challenge. The possibility that IgA antibodies have a role in suppressing hypersensitivity reactions at mucosal surfaces is discussed.