Secretory lysosomes constitute a heterogeneous organelle of hematopoietic cells that combines the properties of regular lysosomes with those of secretory granules. Although secretory lysosomes serve essential functions, such as in the immune system and blood clotting, the mechanisms underlying the release of contents are incompletely understood. It is clear, however, that rab27a and the C2 domain protein munc13-4 serve essential functions. Mutations in these genes lead to immune disorders where the lytic granule function of cytotoxic T cells is jeopardized in humans. We identified munc13-4 as a rab27a binding protein from spleen. Munc13-4 is highly expressed in several hematopoietic cells including cytotoxic T cells and mast cells. We describe the molecular features of the interaction and requirements for localization, and show that munc13-4 is a positive regulator of secretory lysosome exocytosis.