Using methods designed for isolation of mutants defective in receptor-mediated endocytosis, a novel L-cell mutant was obtained that exhibits resistance to three different protein toxins as well as alterations in secretion. This mutant, LEFIC, is resistant to modeccin, Pseudomonas exotoxin, and ricin. These toxins, which enter the cytoplasm via receptor-mediated endocytosis, are thought to penetrate into cells at the level of late endosomes or the trans Golgi network. Early endosomal acidification appears to be normal in the mutant based on its accumulation of iron from transferrin and its sensitivity to diphtheria toxin A chain-transferrin conjugate. Within the secretory pathway two delays in transport of vesicular stomatitis virus (VSV) G protein were observed in LEFIC: a 20-30 min delay in acquisition of Endo H resistance and a 1-2 hr delay in appearance of newly synthesized G protein on the cell surface. Movement of endogenous proteins along the secretory pathway was also affected in LEFIC. Fibronectin secretion was delayed by 15 min, and membrane proteins were delayed in arrival at the cell surface. The phenotype of LEFIC is consistent with a defect in a component or compartment shared by both the late endocytic and constitutive secretory pathways.