We used immunohistochemical staining with antibodies against the novel protein striatal enriched phosphatase (STEP) to investigate the internal organization of grafts of embryonic striatal tissues implanted in the ibotenic acid-lesioned neostriatum of adult rats. STEP immunoreactivity was found in discrete patches within the grafts, which colocalized with areas designated as 'patch' zones when stained for the enzyme acetylcholinesterase and with antibodies against tyrosine hydroxylase and DARPP-32. As previously hypothesized, the pattern of STEP immuno-reactivity in embryonic striatal tissue grafts provides further indication that the patch zones are indeed comprised of striatal like cell populations. The novel protein STEP provides a sensitive and precise marker for this compartment within the grafts.