GD3 is the ganglioside most abundantly expressed on the cell surface of human melanoma, and treatment with a monoclonal antibody recognizing GD3 has induced major responses in a small proportion of patients. However, we have been unable to induce production of GD3 antibodies in melanoma patients by active immunization with GD3-expressing melanoma cells or purified GD3. In this report we describe attempts to increase the immunogenicity of GD3 in the mouse by chemical modification. GD3 lactone I and II, GD3 amide and GD3 gangliosidol were synthesized, and the humoral immune response to these derivatives was compared with the response to unmodified GD3. The GD3 derivatives were more immunogenic than GD3. At a low dose all congeners induced an IgM response, with antibody titers higher than those elicited by low-dose GD3. The gangliosidol and amide derivatives also induced an IgG response. IgM antibodies induced by immunization with GD3 lactone I cross-reacted with purified GD3 and GD3-expressing melanoma cells. Titers of GD3 cross-reactive antibodies were slightly higher than after immunization with GD3 itself at the same low dose. IgM and IgG antibodies induced by the other congeners did not cross-react with GD3.