Leucocyte influx into rabbit ileal loops, induced by strains of Salmonella typhimurium of different virulence, was assessed with 111Indium-labelled leucocytes. Strains fell into two groups on the basis of their leucotactic potential: "virulent" strains (which induced fluid secretion) caused a dose-dependent leucocyte influx; strains which did not induce fluid secretion failed to induce a significant leucocyte influx. Fluid secretion was never observed in the absence of leucocyte influx, but leucocyte influx per se did not induce fluid secretion. The phenotype of the challenge inoculum influenced fluid secretion; young log-phase organisms induced fluid secretion with a higher frequency than overnight cultures. These findings support earlier evidence implicating leucocytes in an interactive but not exclusive role in the genesis of salmonella-induced fluid secretion. They suggest, though do not prove, that interaction of leucocytes with the appropriate phenotype of organisms results in the release of a host-derived or bacterial secretagogue, or both. The bacterial factor may or may not be the antigen related to cholera toxin, described previously.