The lytic proteins mediating target cell killing are stored in the lysosomes of activated cytotoxic T lymphocytes (CTL) and are secreted upon recognition of a target cell. These secretory lysosomes cannot be detected in resting T lymphocytes. Interaction of a resting cell with a target cell activates de novo formation of secretory lysosomes. CTL clones in culture mimic this behaviour, and so provide an ideal system for studying secretory lysosome biogenesis and maturation. In the genetic disease, Chediak Higashi syndrome (CHS), all lysosomes in the cells are enlarged and reduced in number compared with wild-type (WT) cells. We have used CTL from this disease to study secretory lysosome biogenesis and maturation. We show that at early stages after activation the secretory lysosomes are identical in WT and mutant cells, and that delivery of proteins to the secretory lysosome along the biosynthetic and endocytic pathways is normal in the mutant cells. With time, the lysosomes in the mutant cells aggregate, become larger and fewer in number and eventually form giant structures. Our results show that the initial steps of secretory lysosome formation are normal in CHS, but that the organelles subsequently fuse together during cell maturation to form the giant secretory lysosomes.