The biosynthesis and secretion of human interleukin-6 (IL-6) was studied in monocyte cultures stimulated with endotoxin. After labeling with [35S]methionine and immunoprecipitation with a specific antiserum one major (24 kDa) and four minor (27.5, 23.3, 22.5 and 21.8 kDa) molecular mass forms of IL-6 could be found in the cells and media. Incubation of monocyte media with sialidase and subsequently with endo-alpha-N-acetylgalactosaminidase, which cleaves Gal(beta 1-3)Gal-NAc from serine or threonine, led to the formation of only two forms of IL-6 with apparent molecular masses of 25 and 21.8 kDa. The latter had an electrophoretic mobility indistinguishable from that of 125I-labeled recombinant human IL-6. The results suggest that human monocyte IL-6 carries O-glycosidically bound carbohydrates with a Gal(beta 1-3)Gal-NAc core to which only sialic acid is bound. Differences in O-glycosylation are the major cause for the molecular heterogeneity of IL-6. A small part of IL-6 (27.5 kDa form) is in addition N-glycosylated. Incubation of monocytes with tunicamycin and 1-deoxymynnojirimycin and treatment of IL-6 with endoglucosaminidase H suggested that the 27.5 kDa form of IL-6 carries at least one N-linked complex-type oligosaccharide chain.