APO‐1 (Fas/CD95), a member of the tumor necrosis factor receptor superfamily, induces apoptosis upon receptor oligomerization. In a search to identify intracellular signaling molecules coupling to oligomerized APO‐1, several cytotoxicity‐dependent APO‐1‐associated proteins (CAP) were immunoprecipitated from the apoptosis‐sensitive human leukemic T cell line HUT78 and the lymphoblastoid B cell line SKW6.4. CAP1–3 (27–29 kDa) and CAP4 (55 kDa), instantly detectable after the crosslinking of APO‐1, were associated only with aggregated (the signaling form of APO‐1) and not with monomeric APO‐1. CAP1 and CAP2 were identified as serine phosphorylated MORT1/FADD. The association of CAP1–4 with APO‐1 was not observed with C‐terminally truncated non‐signaling APO‐1. In addition, CAP1 and CAP2 did not associate with an APO‐1 cytoplasmic tail carrying the lprcg amino acid replacement. Moreover, no APO‐1‐CAP association was found in the APO‐1+, anti‐APO‐1‐resistant pre‐B cell line Boe. Our data suggest that in vivo CAP1–4 are the APO‐1 apoptosis‐transducing molecules.