The receptors for tumor necrosis factor alpha (TNF alpha) were analyzed on myeloid cells (HL60, U937, K562, and freshly isolated blood monocytes) and on cells of epithelial origin (MCF7, HEp2 and HeLa cells), by use of radiolabeled TNF alpha and cross-linking experiments. Both cell types had high but slightly different affinities for TNF alpha. The myeloid cells had major cross-linked products of 98-100 kDa, which were similar in their N-linked glycosylation, whereas the cells of epithelial origin contained a major cross-linked product of 75 kDa, a second product of 95 kDa. The major receptors of both cell types (studied mostly with HL60 and HEp2 cells) are different proteins because (a) their apparent molecular masses were different and no evidence was obtained for cell-specific proteases, which could generate the differently sized receptors from one common receptor molecule; (b) anti-receptor antibodies, which precipitated the 95- and 75-kDa products, did not precipitate the 100-kDa cross-linked complex; (c) the native TNF alpha-receptor complexes had different proteolytic fingerprints; (d) the tryptic fragments differed in their association with the cell membrane vesicles; (e) the receptors differed in their degree of N-linked glycosylation; and (f) O-linked glycosylation was found on the major receptor of HL60 but not of HEp2 cells. In addition, myeloid cells may also contain a small amount of the HEp2-type of TNF alpha receptor. We suggest that at least two different receptors for TNF alpha exist.