Afipia felis is one of the putative agents of cat scratch disease (CSD). Its intracellular location was previously demonstrated in tissue sections. It has been recently grown in human monocytes and in HeLa cells. We report that A. felis may infect human macrophages and the P 388 D1 murine macrophage cell line, yet is unable to grow in the cell-free culture medium. In both human macrophages and P 388 D1, the organisms multiply in a vacuole after active inhibition of phagosome-lysosome fusion (P-L) as demonstrated with cationized ferritin-labeled lysosomes. This P-L fusion inhibition is diffusable and is due to a filterable factor which is inactivated by protease digestion. The pathogenic role of A. felis in CSD may result from its ability to invade and multiply in macrophages.