A group of brain acid-soluble proteins (BASPs) is preliminarily characterized. In some respects BASPs are similar to high mobility group (HMG) proteins, but in contrast to HMG, all BASPs are very acidic (pI 4.4-4.6) and show abnormal mobility during SDS-PAGE electrophoresis. BASP2-1 and BASP2-2 are identified as the two forms of neuronal protein GAP-43 (B-50, pp46, F1, neuromodulin). BASP1 and BASP3 are apparently novel brain proteins.