GABA-modulin (GM), a basic protein which regulates GABA receptor binding to post-synaptic receptors, has been purified from rat brain synaptosomes and biochemically characterized. Although GM presents some similarities with small myelin basic protein (SMBP), it is biochemically and functionally distinct from any known myelin basic protein. To locate GM in brain, monoclonal and polyclonal antibodies have been produced; one polyclonal antiserum was 4000 - fold specific for GM as opposed to SMBP and detected a single immunoreactive molecular species in rat brain. Granule cells in primary culture, which express the GABA-BZ-C1-receptor/ionophore complex, also contain a protein which is identical by immunological, biochemical and functional criteria to GM isolated from rat brain synaptosomes.