Quantitative microcomplement fixation assays have been used to partially characterize a set of nerve-specific membrane antigens. Their sensitivity to proteolytic enzymes suggested that most, or all, of the antigens contain protein, and the stimulatory effect of neuraminidase treatment suggested that some were glycoproteins. The antigens were found to be approximately 100 times more concentrated in neuronal tissues than in nonneural tissues, except for the adrenal, and they were found to be absent from the forebrains of nonavian species. The changes in total and specific activity have been measured during development in a number of neuronal tissues and compared with previously obtained immunohistochemical data.