Considerable evidence supports a role for polyclonal serum natural Ab (NAb) as a mediator of natural resistance against tumors. However, the molecular mechanisms of this NAb activity are not known. Flow cytometry selection of L5178Y-F9 murine T lymphoma cells for high NAb binding provided the variant LYNAb+, which exhibited an inversely corresponding reduction in tumorigenicity. Accompanying the increased NAb binding, LYNAb+ bound more monoclonal 14.8 anti-CD45RA and DNL-1.9 anti-CD45RC and less 13/2 anti-pan CD45, and the binding of MB23G2 anti-CD45RB was eliminated. However, neuraminidase treatment increased NAb binding and detection of pan CD45, CD45RA, and CD45RC but reduced CD45RB expression, suggesting that the epitopes recognized by the former Abs are masked by sialic acid, while the latter includes sialic acid. Growth of the LYNAb+ from a threshold s.c. inoculum in syngeneic DBA/2 mice yielded more tumorigenic cells which bound less NAb, anti-CD45RA, and anti-CD45RC; the same very low anti-CD45RB; and more anti-pan CD45. In accord with the mAb binding, the L5178Y-F9 and an in vivo passaged LYNAb+ variant expressed predominantly lower m.w. CD45 isoforms while the LYNAb+ expressed predominantly higher 200-kDa isoforms. The consistent correspondence between CD45RA and CD45RC determinant expression, CD45 isoform expression, tumorigenicity, and NAb binding exhibited by T lymphoma cells selected for high NAb binding in vitro or through tumor progression in vivo suggests that asialo high m.w. isoforms of the cell surface-signaling molecule CD45 are differentially expressed during tumor development and furthermore that they participate in NAb-mediated antitumor mechanisms.