Abstract The monoclonal antibody INO (inhibitor of neurite outgrowth) has been shown to bind to a complex of laminin and a heparan sulfate proteoglycan and to block the action of this complex in promoting neurite outgrowth. We now report that the same antibody binds to cytoplasmic constituents in rat adenohypophyseal gonadotropes, as well as to vasopressinergic neurons in the hypothalamus and their terminals in the neurohypophysis. INO immunoreactivity in fixed sections of pituitary does not colocalize with the immunoreactive laminin in blood vessels and glandular basement membranes, although when unfixed tissue is washed in buffer prior to fixation, the INO immunoreactivity appears in these laminin-rich structures. These observations suggest similarities between the INO hypophyseal antigen and the neurite-promoting proteoglycan complex characterized in conditioned media. Presence of this complex in specific neurosecretory cell types suggests that it is involved with specific secretory products with function yet to be determined.