The effect of human gamma interferon (HuIFN-gamma) on the invasiveness of Salmonella typhimurium, Shigella flexneri and Yersinia enterocolitica serotype 0:3 in HEp-2 cell cultures was examined. The intracellular and extracellular bacteria were identified by a combination of Nomarski differential interference contrast microscopy and UV incident light microscopy applied on the same microscope. Pretreatment of HEp-2 cells with HuIFN-gamma reduced in a dose dependent manner the number of S. typhimurium bacteria per cell and the proportion of cells containing bacteria. Maximum inhibitory effect was observed with approximately 10 units per ml of HuIFN-gamma. The inhibitory effect of interferon as well as the antiviral effect was eliminated when the preparation was neutralized with monoclonal anti-interferon globulin and was acidified to pH 2. Murine gamma interferon did not influence invasiveness of S. typhimurium. No effect of interferon was observed when the bacteria were incubated in cell cultures at 4 degrees C. Invasiveness of S. flexneri and Y. enterocolitica was not influenced by treatment of cells with HuIFN.