Solutions of human secretory IgA (S-IgA) did not interact with complement. When S-IgA solms were lyophilized, and resuspended with gentle stirring only 50-70 per cent of the resulting powder dould be redissolved subsequently. Incubation of the insoluble residue with human serum remove complement activity. Simple freezing and thawing of S-IgA solns did not produce anticomplementary activity. Solution of human S-IgA, IgG, and albumin were evaporated on a rotary evaporator at 37°C. The residues were resuspended and analyzed for anticomplementary activity using a method which measures rate of cell lysis by following turbidity changes in suspensions of sensitized red cells to which complement has been added. On a relative scale, the quantities of the individual human residues required to remove one half of the complement offered were: IgG = 1, IgA = 5, albumin = 27. Aggregated human Fabα fragments of secretory IgA as well as aggregated serum myeloma IgA were also found to be complement reactive. Aggregated secretory IgA was found to reduce the hemolytic activity of human C3-9 but not that of human C1. the IgA aggregates were probably not held together with covalent bonds since they readily dissolved at alkaline pH. The results suggest that the Fab region of S-IgA may be involved in the complement interactions of S-IgA aggregates and that secretory component is not necessary of these interactions. © 1974.