Two different Type I interferon receptors (IFN-R) have been described: the normal and the variant receptors. The alpha subunit of the Type I IFN-R has a molecular mass of 110 kDa in cells expressing normal and variant receptors. The beta subunit has a molecular mass of approximately 100 kDa in cells that express normal receptors and 55 kDa in cells expressing the variant form of the receptor. The IFN alpha-resistant U-937 cell line expresses variant receptors and fails to down-regulate and phosphorylate the alpha subunit on tyrosine residues. We report that two other myelomonocytic cell lines, YK-M2 and ML-2, also expressing the variant form of the receptor, fail to down-regulate and phosphorylate the alpha subunit on tyrosine residues. However, YK-M2 and ML-2 cells are sensitive to the antiproliferative and antiviral effects of IFN alpha 2, indicating that phosphorylation of the alpha subunit is not necessary to elicit an IFN alpha response and that expression of variant receptors is not a source of IFN alpha resistance. We also determined if other proteins involved in the IFN alpha signal transduction pathway had a different phosphorylation pattern. Treatment of cells expressing variant receptors induced tyrosine phosphorylation of the p135tyk2 tyrosine kinase, and the three interferon-stimulated gene factor 3 alpha (ISGF3 alpha) polypeptides (p113, p91, and p84), albeit at lower levels. These results indicate that cells expressing either form of the Type I IFN-R phosphorylate a similar set of proteins, with the exception of the alpha subunit.