Vaccination with irradiated, GM-CSF–secreting RENCA cells stimulates potent anti-tumor humoral immunity. Sera were collected from syngeneic Balb/c mice that were either naive or immunized 10 times at weekly intervals with irradiated GM-CSF–secreting or parental RENCA cells. (A) Sera were diluted 1:100 and evaluated for reactivity against live RENCA cells by flow cytometry. An anti–mouse pan-IgG secondary antibody was used to detect isotypes that depend upon CD4⁺ T cell help for class switching. (B) IgG2a and IgG1 are the main isotypes stimulated by vaccination. (C) Sera obtained after vaccination mediates more potent complement-dependent lysis of RENCA cells than sera from naive mice. Representative of 4 independent experiments.