By using the radiolabeled immune complex method for studying phagocytosis by neutrophils, release of the following lysosomal enzymes has been demonstrated: the C5-cleaving enzyme, β glucuronidase, and acid phosphatase. Enzyme release is directly proportional to cell uptake of immune complexes and is suppressed in antigen excess. It has also been shown that human IgM rheumatoid factor, but not IgM lacking in rheumatoid factor activity, inhibits phagocytic uptake of immune complexes and, correspondingly, depresses the release of lysosomal enzymes.