Apoptosis, programmed cell death, was previously shown to be induced by the mAb anti-APO-1 (IgG3, kappa) by binding to the APO-1 cell surface Ag, a new member of the nerve growth factor/TNF receptor superfamily. To investigate the role of the Ig H chain Fc regions we compared induction of apoptosis by the original mAb IgG3 anti-APO-1 with anti-APO-1 F(ab')2 fragments and different anti-APO-1 isotypes (IgG1, IgG2b, IgG2a, and IgA) isolated by sequential sublining. We found that IgG3 was the most active isotype; IgG1, IgG2a, and IgA showed intermediate activity, and IgG2b and F(ab')2 were inactive. Cytotoxic activity of the inactive or less active antibody preparations was fully reconstituted by protein A, anti-mouse Ig, or anti-mouse Ig F(ab')2, respectively. Thus, APO-1-mediated induction of apoptosis was dependent on efficient cross-linking of APO-1 cell surface Ag, indirectly augmented by anti-APO-1 Fc-Fc self-aggregation. Because of their different in vitro activity we selected IgG3-, IgG2b-, and IgA anti-APO-1 to test their antitumor activity against solid human B lymphoblastoid tumors in SCID mice. The isotypes showed a different serum half-life (IgG3: 9.2-10.4 days, IgG2b: 1.9-2.6 days, and IgA: 14.1-29.2 h) and a different initial tumor localization 4 h after i.p. injection (IgG3 around the blood vessels, IgG2b homogeneously, and IgA heterogeneously distributed in the tumor). All antibody preparations induced tumor regression by induction of apoptosis, even IgG2b anti-APO-1 inactive in vitro without cross-linking. The activity of IgA anti-APO-1, which did not mediate complement-dependent cytotoxicity or antibody-dependent cellular cytotoxicity indicates that apoptosis may be used as the main if not the only mechanism of induction of tumor regression in vivo. As with in vitro, IgG3 anti-APO-1 was the most effective isotype also in vivo. This result suggests that cross-linking of APO-1 on the tumor cell surface may also be required for tumor regression by apoptosis in vivo. Taken together, our data show that selective targeting of apoptosis to tumors may be an efficient antitumor mechanism.