The cytotoxic effects of penicillic-acid (90653) (PA) were tested in cultured alveolar cell macrophages harvested from male Long-Evans- hooded-rats by tracheal lavage. There was significant release of chromium-51 from labeled cells after 2 hours of exposure to 1 millimolar (mM) PA, but not after 2 hours; the levels of adenosine- triphosphate in the culture declined significantly after 4 hours of exposure. Dose dependent inhibition of tritiated leucine into proteins and of protein synthesis after 2 hours of exposure to 2mM PA or more was observed. Inhibition of RNA synthesis was less pronounced than that of protein synthesis at 1mM PA after 4 hours, and concentrations lower than 1mM did not affect this parameter. After 2 hours of exposure, the median effective doses for protein and RNA synthesis inhibition were 0.18 and 0.60mM, respectively. The median effective dose of PA on phagocytosis, which was inhibited by exposure to doses equal to or greater than 0.03mM, proved to be lower than those of all the other parameters studied and amounted to macrophages and suggest that the agent may be a hazard to agricultural workers exposed to contaminated grains.