Heat or chemically aggregated IgA or IgG stimulated degranulation of neutrophils with comparable efficiency. The same aggregates induced a neutrophil respiratory burst which could be measured by lucigenin-enhanced chemiluminescence. Serum IgA1 or IgA2 coated onto microtitre plates were both capable of inducing a respiratory burst in neutrophils, as was secretory IgA1 or secretory IgA2. All bursts were of similar size for a given concentration of IgA and were greater than the burst elicited by an equivalent concentration of IgG. For each subclass of IgA the respiratory bursts were dependent on their density on the opsonized surface. Since monomeric and dimeric forms present in secretory IgA preparations both elicit a respiratory burst in neutrophils, secretory component and J chain cannot block the receptor binding site on the Fc region. The potential of secretory IgA to act as an opsonin might have important consequences on mucosal surfaces where the availability of complement components is limited.