We have previously reported the production of a new monoclonal antibody (MAb) (SP-2) recognizing a 90-kDa tumor-associated antigen, termed 90K, which is increased in the serum of many cancer patients. Treatment of CG5 human breast cancer cells with recombinant interferon-alpha 2b (rIFN-alpha 2b) can increase the synthesis and release, in culture medium, of the 90K. The effect of rIFN-alpha 2b was dose-related and occurred at concentrations which did not affect cell proliferation. The increase of 90K expression was due to de novo protein synthesis since cycloheximide, added within 3 hr of the beginning of rIFN-alpha 2b stimulation treatment, completely abolished the effect of rIFN-alpha 2b. The stimulatory effect of rIFN-alpha 2b was already evident after 24 hr treatment. Finally, an increase in serum 90K levels was observed in 3 patients with advanced breast cancer receiving a short course of rIFN-alpha 2b (Intron A). No effect of rIFN-alpha 2b was seen in 3 normal women. The ability of rIFN-alpha 2b to increase the synthesis and release of 90K both in vitro and in vivo may be of clinical importance in the early detection of tumors.