P48 is a recently described 48-kDa differentiation-inducing cytokine isolated from the culture medium of the human leukemia line Reh. P48 induces differentiation and cytolytic activity in the promyelocytic cell line HL-60, and stimulates the release of TNF-alpha and IL-1 from peripheral blood monocytes. In further studies designed to examine the biosynthesis and function of P48, surface immunofluorescence flow cytometry analysis as well as 125I surface labeling and immunoprecipitation, revealed the presence of P48 on the surface of Reh cells. Triton X-114-treated Reh cells were partitioned into detergent and aqueous phases and separated by SDS-PAGE. Western blot analysis revealed that P48 partitioned exclusively into the detergent phase, suggesting an integral membrane association. Reh cells fixed with paraformaldehyde, but not K562 or P815, were able to stimulate the release of TNF-alpha from peripheral blood monocytes in a manner similar to that of secreted P48. Isolated plasma membranes from Reh cells could also stimulate TNF-alpha release. This TNF-alpha-releasing activity could be removed from detergent solubilized Reh membranes by immunoaffinity chromatography on an anti-P48 column. This study suggests that, in addition to being secreted into the culture medium, P48 is expressed on the surface of Reh cells in a biologically active form. The membrane form of P48 may be 1) a final maturation step before secretion or 2) a cell membrane-associated form that may be analogous to the membrane forms of TNF-alpha and IL-1.