Pseudomonas exotoxin A enters mouse LM fibroblasts by receptor-mediated endocytosis and ultimately causes cell death. Here we present evidence for the existence of a specific receptor for the toxin. Toxin association with LM cells at 18 and 37 degrees C, but not at 4 degrees C, was highly specific. At 37 degrees C, the association increased with time, reaching a steady state by 5 h. Binding to paraformaldehyde-fixed cells at 37 degrees C was saturable (Kd = 5.4 nM), was reversible, and indicated ca. 100,000 binding sites per cell. It is believed that receptor-bound toxin is responsible for cell death. Once the kinetics of toxin entry were described, we examined the effect of reduced temperatures on the intracellular processing of toxin and thus its expression. Toxin-induced inhibition of protein synthesis was minimal at temperatures below 20 degrees C. This was seen even though at 20 degrees C sufficient toxin was internalized to kill cells, and toxin enzyme activity was maximal. Internalization of 125I-labeled toxin, but not of 125I-labeled horseradish peroxidase (marker of fluid-phase endocytosis), became rate limiting at 20 degrees C or below. These data suggest that reduced temperatures block a step in the receptor-mediated endocytic pathway essential for the expression of Pseudomonas toxin activity.