Evidence of a local immune activation in cystic brain tumors. The fluid of cystic brain tumors was characterized with regard to the protein content. In most malignant tumors, the concentrations of immunoglobulins G and M (IgG and IgM) were higher relative to other proteins in the cyst fluid than in the serum of the same patient. A markedly elevated ratio of monomeric to pentameric IgM was detected in the cyst fluid of two patients with glioblastomas. The results indicate a local immunoglobulin synthesis in malignant cystic brain tumors. It is hypothesized that higher-than-expected concentrations of IgG and IgM in cyst fluid as compared to plasma are a sign of an ongoing immune response triggered by the tumor. 