L1210 leukemia cells grew progressively and caused tumor deaths in all recipient mice. However, when these cells had been treated with Vibrio cholerae neuraminidase (VCN) prior to injection, tumor deaths did not occur. Both untreated and VCN-treated L1210 cells elicited a humoral response, as manifested by an increasing percent of cells in the spleen and peritoneal cavity, with various types of membrane-associated immunoglobulins. Progressive tumor growth was associated with a large percent of peritoneal exudate (PE) cells bearing membrane-associated IgG, a late increase in the percent of PE cells with IgG, and only a small percent of PE cells with IgM on their surfaces. Conversely, PE cells from mice given VCN-treated L1210 cells were characterized by a small percent with IgG, an early increase in percent of cells with IgG, and a large percent with membrane-associated IgM. An injection of VCN-treated L1210 cells into mice with progressively growing L1210 tumors caused frequent tumor remissions, with a corresponding alteration of the ongoing humoral responses. Both the degree of alteration and the number of cures depended on the tumor burden at the time VCN-treated tumor cells were injected. The humoral response in mice with tumor remission following immunization was comparable to the response detected after an injection of VCN-treated cells only.