The integrity of the host cell may represent an important prerequisite for the intracellular survival and development of obligate intracellular pathogens. In the present study, we investigated the influence of infections with the protozoan parasite Toxoplasma gondii on the rate of apoptosis in the human leukemia cell line HL-60. After infection with T. gondii tachyzoites of the strain NTE and in uninfected controls, less than 2% of the host cells showed typical signs of apoptosis, i.e. condensation of chromatin after staining with Hoechst 33258 or internucleosomal DNA fragmentation after agarose gel electrophoresis of genomic DNA. After treatment with 0.1 to 0.5 microg/ml actinomycin D for up to 16 hours, HL-60 cells considerably underwent apoptosis. However, this actinomycin D-induced apoptosis was clearly reduced after concomitant infection with T. gondii as shown by staining with Hoechst 33258 and by DNA fragmentation assay. Inhibition of apoptosis by the intracellular pathogen T. gondii might be recognized as an evasion mechanism that enables intracellular survival and establishes long-lasting persistence.