This article presents evidence that protein kinase activity is an intrinsic property of secretory immunoglobulin A (sIgA) from milk of healthy human mothers. Polyclonal sIgA was purified by sequential chromatography on protein A-Sepharose, DEAE-cellulose, and gel filtration on Toyopearl HW-55 and Sepharose 4B columns. Its purity was established by one- and two-dimensional SDS-PAGE. The protein kinase activity was inhibited by specific antibodies (Abs) against sIgA, and was stable to acidic and alkaline conditions. Catalytic sIgA showed optimal reaction conditions (pH and MgCl2 concentration) and substrate specificity different from those of known protein kinases; i.e., sIgA phosphorylated the serine residues of various milk proteins in the presence of different gamma-[32P]nucleoside- and deoxynucleoside-5'-triphosphates. The homogeneous Fab fragment of sIgA also showed kinase activity. An ATP-binding activity of fractions of sIgA was demonstrated by affinity chromatography on ATP-Sepharose and by covalent binding of an affinity analog of ATP; this activity was mediated by the L chain of sIgA. The authors believe these observations are the first example of the catalytic activity of IgA Abs and of natural catalytic Abs with synthetic activity. In addition, the findings suggest the likelihood that catalytic Abs are generated by the immune system of healthy mothers.