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Monoclonal nonspecific suppressor factor (MNSF) is a lymphokine product of a murine T cell hybridoma that inhibits the immune response in an antigen nonspecific manner. Recently, we found that a novel ubiquitin-like protein (Ubi-L), a subunit of MNSF, is responsible for its biological activity. We developed a monoclonal antibody with specific activity against Ubi-L. Inhibition experiments showed that this mAb, termed NA4, preferentially recognizes Ubi-L but not irrelevant proteins such as ubiquitin. With the use of NA4, we established an ELISA method for the quantitation of Ubi-L. By this ELISA system, approximately 40 ng/ml of MNSF was detected in the culture supernatants of concanavalin A (Con A)- or interferon gamma (IFN gamma)-activated splenocytes, whereas MNSF in the supernatant of IFN alpha- and IFN beta-stimulated splenocytes was nil. In addition, NA4 could abrogate the action of Ubi-L. Thus NA4 was confirmed to be a pertinent tool for elucidation of the underlying mechanism of action of MNSF.