Human interleukin-6 (IL-6) secreted by cytokine- or endotoxin-induced fibroblasts, monocytes, keratinocytes, endometrial stromal cells, and endothelial cells, when analyzed under denaturing and reducing conditions, consists of a set of differentially modified phosphoglycoproteins of molecular mass in the range from 23 to 30 kD (a set of at least three O-glycosylated 23- to 25-kD species and a set of at least three N- and O-glycosylated 28- to 30-kD species). The 23- to 25-kD and 28- to 30-kD fibroblast-derived IL-6 species have been separately purified to homogeneity with the use of a combination of lectin and immunoaffinity chromatography. Glycosidase digestion experiments on such purified preparations confirmed that almost all human fibroblast-derived IL-6 species were O-glycosylated; additionally, the 28- to 30-kD species were N-glycosylated. Amino acid sequencing revealed that the major amino terminus in the fibroblast-derived 23- to 25-kD O-glycosylated IL-6 was at Ala28 whereas the major amino terminus in the 28- to 30-kD N- and O-glycosylated IL-6 was at Val30, suggesting that targeting of newly synthesized IL-6 polypeptides into the two different processing pathways in fibroblasts may be keyed to differences in the signal peptide cleavage site. Unexpectedly, IL-6 "constitutively" secreted by the Epstein-Barr virus (EBV)-infected human and primate (tamarin) B-cell lines designated sfBJAB and sfBT, respectively, consisted of a major apparently unglycosylated 21-kD species and a minor 25-kD N-glycosylated species.(ABSTRACT TRUNCATED AT 250 WORDS)