Complement receptor 3 (CR3), one of cell adhesion molecules, plays a crucial role in secretion of mammalian neutrophils. To study whether CR3 is also involved in neurosecretion of gastropod molluscs, we examined the CR3-like immunoreactivity in the central nervous system (CNS) and the superior and inferior tentacles of the terrestrial slug, Limax marginatus. In the CNS of L. marginatus, we did not detect the specific immunoreactivity at all. In contrast, the CR3-like immunoreactivity was observed in the cell bodies and processes of collar cells of the superior and inferior tentacles. In particular, granules contained in the cell bodies of collar cells exhibited the CR3-like immunoreactivity. Retrograde labeling of horseradish peroxidase applied on the sensory epithelia (SE) of superior and inferior tentacles showed that the collar cells of these tentacles project their processes to the SE. In the previous study, homogenate of the superior tentacles injected into the body cavity of slugs stimulated spermatogenesis and simultaneously inhibited egg-laying behavior, suggesting that the tentacular hormone acts as gonadotropic hormone. Taken together, CR3 expressed in the collar cells is considered to be involved in the secretion of tentacular hormone in L. marginatus.