We studied three patients in whom all or part of their serum alkaline phosphatase circulated as a complex with immunoglobulin G (IgG). Serum alkaline phosphatase isozymes were visualized by their electrophoretic (in agar, agarose, and starch gel and on cellulose acetate), gel filtration, and electroimmunodiffusion behaviour. The alkaline phosphatase-IgG complex was of the liver type (two cases) and bone type (one case). The reaction pattern of alkaline phosphatase with different human tissues and with sera of heterologous origin suggests that the complex is of the antigen-antibody type. A direct genetic mechanism seems unlikely since in one patient the IgG-alkaline phosphatase complex was not present in previous serum samples. The presence of this complex has no apparent correlation with the observed abnormalities. Alkaline phosphatase linked to immunoglobulin G must be considered in the interpretation of increased serum alkaline phosphatase.