BALB/c mice immunized with Nonidet P-40 (NP-40) crude solubilized (CS) extracts of a syngeneic methylcholanthrene-induced BALB/c sarcoma (Meth A) were challenged with viable Meth A cells to determine the ability of the solubilized preparations to induce transplantation rejection. Animals resisting such challenge were then used in agarose microdroplet macrophage migration inhibition (MMI) and tumor cell neutralization (Winn) assays to evaluate the antigenic specificity of these CS extracts. Spleen cells from those animals that rejected Meth A after immunization with the NP-40-solubilized preparations effectively neutralized the tumorproducing capacity of Meth A tumor cells as determined in Winn assays. MMI assays were quite sensitive and detected migration inhibition of peritoneal exudate (PE) cells from immunized mice with extract concentrations as low as picogram quantities. Specificity studies demonstrated that Meth A expressed no antigenic cross-reactivity with similarly prepared extracts of an unrelated SV40-induced sarcoma (mKSA), nor with a mineral oilinduced plasmacytoma (ADJ-PC5) of BALB/c mice. Inhibition of PE cell migration was mediated by culture supernatants (presumably migration inhibition factor [MIF]) generated from a mixture of immune spleen cells and mitomycin C (MMC)-treated Meth A cells as assayed in an indirect MMI test.