The low density lipoprotein receptor-related protein (LRP) is a large multifunctional receptor implicated in the cellular uptake of functionally diverse ligands. Biochemical evidence suggests that LRP is a clearance receptor for apoE-rich remnant lipoproteins, lipoprotein lipase, α₂-macroglobulin/protease complexes, plasminogen activator/inhibitor complexes, the active protease tissue-type plasminogen activator and exotoxin A from Pseudomonas aeruginosa. Mice genetically deficient in LRP die early during gestation, underscoring the essential physiological role of this gene in vivo. To study the effect of LRP deficiency at the cellular level, we have used Pseudomonas exotoxin A (PEA) to select murine embryonic fibroblasts that are genetically deficient in LRP. Our results demonstrate that this single gene defect is sufficient to confer resistance to PEA on cultured cells. In addition, embryonic fibroblasts lacking LRP are unable to bind, internalize and degrade methylamine-activated α₂-macroglobulin and complexes of urokinase with plasminogen activator inhibitor-1. Furthermore, cellular uptake and degradation of receptor-associated protein, a 39 kDa accessory protein of LRP, is reduced by 90% in the absence of LRP. These results provide genetic evidence for the multifunctional nature of LRP and its crucial role in protease/inhibitor complex metabolism.