Pseudomonas exotoxin (PE) was used as a probe to study the mechanism by which protein ligands are internalized by mammalian cells. Both biochemical and electron microscopic methods were used to look at the internalization of PE by mouse LM cell fibroblasts. Our data suggest that PE enters cells by receptor-mediated endocytosis, a process previously thought to be restricted to the entry of biologically significant molecules such as lysosomal enzymes and peptide hormones. Biochemical studies showed that methylamine (20 mM) and chloroquine (10 microM) protected LM cells from the action of PE. Full protection was observed if methylamine or chloroquine was added to the monolayers simultaneously with toxin or if they were added up to 10 min after toxin binding. Later addition of amine or chloroquine afforded partial protection to the monolayers. With immunoelectron microscopy we observed that in the cold toxin bound diffusely to the cell surface but was rapidly internalized when cells were warmed to 37 degrees C. In the presence of methylamine, chloroquine or ammonium chloride, internalization did not occur. We propose that PE enters mouse fibroblasts by receptor-mediated endocytosis and that chloroquine and methylamine, agents which are known to block this process, prevent expression of toxicity.