Abstract: Previous studies showed that levels of some glycosphingolipids (GSLs) expressed in solid brain tumors grown in vivo were reduced or undetectable in cultured cells prepared from the tumors. This phenomenon has been attributed either to suppressed glycolipid synthesis from unknown forces of the tissue culture environment or to the absence of host cells that normally infiltrate the solid tumors growing in vivo. To test further the host cell hypothesis, we examined host cell markers in two experimental mouse brain tumors, the ependymoblastoma and the CT-2A, that were grown as subcutaneous solid tumors in the flank of C57BL/6J (B6) mice or as cultured cells in vitro. The markers included ganglioside N-glycolylneuraminic acid (NeuGc), GA1 (asialo-GM1), and Fc receptor-bearing cells. NeuGc-containing gangliosides, GA1, and Fc receptors are expressed by macrophages and lymphoid-type cells of the mouse host immune system but are not normally expressed by mouse neural cells. Differences in the relative content of Fc receptor-bearing cells in ependymoblastoma and CT-2A tumors grown in vivo (8.3 and 16.8%, respectively) were proportional to differences in the relative content of NeuGc-containing gangliosides (25.5 and 45.1%) and GA1 (8.5 and 13.8%), respectively. Neither cultured tumor cell line expressed Fc receptors, GA1, or NeuGc-containing gangliosides. These findings suggest that non-neoplastic host infiltrating cells (macrophages) contribute significantly to the GSL composition of solid tumors growing in vivo.