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This review will focus on the complement receptor type 1 (CR1, CD35) expressed by erythrocytes and will cover its structure, molecular biology, and function as a membrane inhibitor of complement activation. The CR1 present on phagocytic cells and lymphocytes has similar functions in regulation of complement activation and serves as a receptor responsible for triggering cellular activation events such as phagocytosis or Ig synthesis. These latter receptor functions of CR1 are not covered in this review, and the reader is referred to several past reviews about leukocyte CR1 for this information (Ross and Medof 1985; Wright and Griffin 1985; Fearon and Ahearn 1989; Ross et al. 1989). Erythrocyte CR1 has three functions in regulation of complement activation that are covered in this review: (a) inhibition of the C3 and C5 convertases of the classical and alternative pathways of complement activation; (b) factor I cofactor activity for cleavage of C3b and iC3b; (c) adsorption of soluble immune complexes, thereby inhibiting complement-mediated inflammation. Table 1 summarizes the attributes of CR1 on all cell types.