Mice inoculated with MCA-10 sarcoma cells which had previously been incubated with Vibrio cholerae neuraminidase (VCN) demonstrated a significantly lower tumor incidence (9/26) than mice injected with untreated sarcoma cells (10/10) or sarcoma cells incubated with heat-inactivated neuraminidase (28/29) p less than .05. Rechallenge of nontumor-bearing mice from the VCN group with untreated sarcoma cells resulted in a low tumor incidence (4/11), indicating that these mice had developed systemic immunity following the initial injection of VCN-treated tumor cells. These mice also demonstrated significant lymphocytotoxicity against MCA-10 target cells compared with normal control mice (p less than .05). Subsequent cytotoxicity experiments, utilizing groin lymph node and splenic lymphocytes from mice five days following leg injection of VCN-treated, heat-inactivated VCN-treated or untreated MCA-10 cells, demonstrated that the mice injected with VCN-treated tumor cells demonstrated greater antitumor immunity both locally and systemically. This magnification of tumor immunity is postulated as the mechanism by which neuraminidase treated MCA-10 sarcoma cells grew less well in C57 mice than cells incubated with heat-inactivated VCN or cells left untreated.