1. Calcium (Ca)-binding proteins of neuronal ganglia and of single, identified neurons of the marine mollusk, Aplysia californica, were investigated. Using transblot/45Ca overlays two proteins, at Mr 45,000 and Mr 23,000, with a high Ca-binding ability were found. 2. Western blot analysis revealed that the protein at Mr 45,000 could be separated by 2D-PAGE into proteins with Mr 40,000 and Mr 43,000. The protein at Mr 40,000 immunocross-reacted with antisera directed against parvalbumin and rat calbindin D-28K, indicating a novel Ca-binding protein sharing common antigenic determinants for both proteins. 3. The protein at Mr 23,000 could be separated into a group of proteins with Mr 13,000-20,000 which showed a high degree of similarity to sarcoplasmatic calcium-binding proteins (SCP). 4. We further investigated the protein pattern of single, identified neurons of different electrical activity (bursting, beating, and silent) by 2D-PAGE. Major differences were found in the range of low Mr and low pI, where Ca-binding proteins are generally located. A protein at high concentrations characteristic for silent cells migrated at a position similar to crayfish SCP. 5. The results show that various Ca-binding proteins are characteristic for neurons in the Aplysia nervous system and support the idea that they may effect the electrical behavior of nerve cells.