Decay-accelerating factor (DAF/CD55) is a glycosylphosphatidylinositol-anchored protein which is known to have signal transducing capacity and to be associated with several proteins. To determine the signal transducer in the DAF-forming complex, we purified DAF-associated proteins from Raji B cells using an anti-DAF mAb (1C6)-bound affinity column and established five mAb against them. Among these, mAb 2E12-G7(IgM/kappa) reacted with a variety of intact cells, including peripheral blood mononuclear cells (PBMC), as well as cells from T and B cell lines, as shown by cytofluorimetric analyses. The Mr of 2E12-G7 antigen was estimated to be 43 kDa by surface biotinylation and immunoblotting analysis. This antigen was demonstrated in 1C6 immunoprecipitates, but not in anti-CD59 (another GPI-anchored complement regulatory factor)-immunoprecipitates. Sequential treatment with 1C6 F(ab')2 and then with anti-mouse Ig F(ab')2 stimulated PBMC to induce tyrosine phosphorylation on proteins of 45, 72, 78 and approximately 100 kDa. Also, mAb cross-linked to 2E12-G7 stimulated PBMC to induce tyrosine phosphorylation on proteins of 72, 78 and approximately 100 kDa. Furthermore, when 2E12-G7 and 1C6 immunoprecipitates were incubated with [gamma-32P]ATP, the main constituents detected in both were phosphorylated proteins of 26, 32 and 62 kDa. Thus, DAF-associated 2E12-G7 antigen transduces a signal, similar to the DAF molecule.