The following observations indicate that cultured human WI-38 fibroblasts synthesize and secrete alpha2-macroglobulin into serum-free medium: (a) after incubation of cultures with [35S]L-methionine, a labeled protein appeared in the medium which was precipitated by antiserum directed against alpha2-macroglobulin; (b) after incubation of cultures with [35S]L-methionine, a major band of radioactivity detected by polyacrylamide gel electrophoresis of the proteins in medium co-migrated with alpha2-macroglobulin; and (c) the amount of alpha2-macroblobulin in the medium, estimated both functionally and immunologically, increased with time in normal but not not puromycin-treated cultures.