Hypersensitivity-like reactions are an infrequent but persistent clinical manifestation of hemodialysis. Although a Limulus-amebocyte-lysate-reactive material (LAL-RM) has been demonstrated in some hemodialyzers, its potential role in causing anaphylactoid reactions has not been addressed heretofore. In this chapter, current knowledge about LAL-RM will be presented and integrated with the major mechanisms generally recognized to induce hypersensitivity-type reactions. These mechanisms include: classical induction of allergy by IgE and classical complement activation by IgG and IgM. Alternative complement activation will be not discussed as it has been extensively reported in the literature of hemodialysis and because LAL-RM has not been shown to activate C3a extensively.