Ten recent clinical isolates of Salmonella serotype Typhimurium from man that were tested for their invasiveness in rabbit ileal explants in vitro, were compared with Typhimurium strain TML, a well-characterised invasive strain isolated from a case of human gastro-enteritis. Nine of the 10 strains showed invasiveness that was comparable to that of strain TML. One isolate (GM3) was apparently substantially less invasive; electron microscopy showed this strain to be histotoxic - the probable reason for its reduced recovery from ileal mucosa and thus apparent 'low' invasiveness. Salmonella serotype Choleraesuis strain A50, isolated from a case of systemic salmonellosis in pigs, and serotype Dublin strain 3246, isolated from a case of systemic salmonellosis in calves, were also examined. Dublin strain 3246, when grown at 37 degrees C and used immediately in the invasion assay, damaged the mucosa in a manner similar to that of Typhimurium strain GM3, whereas Dublin strain 3246 grown at 37 degrees C and stored overnight at 4 degrees C did not. This was reflected in an apparently lower invasiveness of freshly grown organisms compared with that of organisms stored at 4 degrees C. In contrast, the histotoxicity of Typhimurium strain GM3 was not affected by storage at 4 degrees C. When stored at 4 degrees C, the levels of invasiveness of Choleraesuis strain A50 and Dublin strain 3246 were not significantly different from each other or from Typhimurium strain TML.