Immunolabelling of Shiga toxin in macrophages infected with a non-invasive Shigella dysenteriae 1 isolate showed that bacteria remained alive for 3 h after ingestion within the phagocytic vacuole and synthesized Shiga toxin. The normal process of toxin secretion was, however, impaired by the phagosomal environment and toxin molecules accumulated within the bacterial cytoplasm.