The COOH-terminal amino acid sequence of the rat brain-specific polypeptide 1B236 was previously deduced from molecular cloning and nucleotide sequence determination of its mRNA and the 1B236 protein shown to exist in the rat brain. The amino acid sequence of 1B236 contained at least three peptide sequences demarcated by pairs of basic amino acids--a structure similar to known neuropeptide and hormone precursors--which suggested that the protein might be processed in vivo to generate peptides. We have developed radioimmunoassays specific for 1B236 with antibodies against three synthetic peptides corresponding to putative cleavage products of this protein and have used these assays to define the molecular forms of 1B236 in rat brain extracts. The most abundant form is of high molecular weight (ca. 100,000) and requires detergent for solubilization; hence, it is probably membrane-bound. However, a small fraction of the high molecular weight material is soluble in the absence of detergent. In addition, several low molecular weight species are detectable in brain extracts prepared under conditions preventing proteolysis. These molecules correspond in size to two of the possible products of proteolytic processing predicted from the amino acid sequence of 1B236. The multiplicity of 1B236 forms, together with other data, suggests that this protein undergoes extensive post-translational modification, including proteolytic processing to generate peptides that may be physiologically relevant.