Cytotoxic activity on macrophage J774A. 1. J774A.1 macrophages grown in 24-well plates for 20 h were infected with strains grown to an OD580nm of 1.0-1.5 (MOI of 5). The cytotoxicity was followed over a 4 h period by measuring LDH release using a cytotoxicity detection kit (Promega). Values are expressed as a mean concentration of LDH in the culture after 4 h of incubation. Data are mean values from three independent experiments.