Contamination of Strain L cell cultures by pleuropneumonia-like organismtis (PPLO) resulted in a complete inhibition of the incorporation of tritiated thymnidine and uridine. Contanminated cultures were characterized by a concentration of PPLO along the margins of the cells and in the intercellular-spaces of the cultures. Autoracliograins of PPLO-contaminated cultures were characterized by exposed silver grains over the margins of the cells. Kanamnycin was efjective in the elimination of PPLO and the restoration of nucleoside incorporation.