ImuVert is a biological response modifier (BRM) that has antitumor effects in humans and rats. The influence of ImuVert on the ganglioside composition of two experimental brain tumors, ependymoblastoma and CT-2A, was studied in C57BL/6J mice. Gangliosides are expressed on plasma membranes and can serve as markers to distinguish neural cells from nonneural cells in mouse brain tumors. N-acetylneuraminic (NeuAc) is the predominant sialic acid in mouse neural cells, whereas N-glycolylneuraminic (NeuGc) is a major sialic acid in nonneural cells, e.g., macrophages and lymphocytes. ImuVert treatment increased the NeuGc ganglioside concentration in the ependymoblastoma, but had no effect on the sialic acid concentration in the CT-2A brain tumor. ImuVert also had a slight inhibitory effect on the growth of both brain tumors.