Serial serological studies were carried out on 19 of 20 patients with malignant gliomas who were actively immunized with one of two human glioma tissue culture cell lines (D-54MG or U-251MG). Most patients mounted a significant serum reaction to histocompatibility antigens (HLA's), as well as an antibody response to fetal bovine serum (FBS) which was added to the glioma-cell inoculum. These two sources of antibody accounted for greater than 90% of the antibody induced by these inoculations. Two patients continued to have significant amounts of binding antibody to the original immunizing cell line following exhaustive absorptions of FBS and HLA antibodies. One of these two had all remaining significant antibody removed by further absorption of the serum against the 2-T osteogenic sarcoma culture cell line known to possess antigens cross-reactive with human gliomas. One single patient continued to show significant antibody binding to the original glioma cell line following absorption against FBS, human platelets, and the 2-T cell line, and therefore seems to have produced glioma-distinctive antibodies in response to immunization. The antibody preparation from this patient was also cytotoxic against the original glioma cell line, as well as another recently cultured human glioblastoma cell line. The significance of these serological studies is discussed as it relates to immunological responses patients with gliomas may make to active immunization.