The integral membrane protein CD20 has been identified as an important therapeutic target in the treatment of non-Hodgkin's lymphoma (NHL). CD20 binding of many antibodies including the therapeutic antibody, rituximab, has been shown to be critically dependent upon the conformation of a loop structure between the third and fourth helical transmembrane regions. In this work, human and murine CD20 proteins expressed in Escherichia coli are shown to be localized with the cell membrane and are purified in nondenaturing detergent solutions. The purified human and murine CD20 proteins have a substantial helical structure as measured by circular dichroism spectroscopy. Only small changes in the secondary structure are observed following the reduction of CD20, with the addition of SDS, or after heating. The rituximab antibody is shown to bind to purified human CD20 with nanomolar affinity. Rituximab binding is abolished by reduction and alkylation of CD20, with data consistent with the proposed antibody epitope being within the disulfide-bonded loop formed between cysteine residues 167 and 183. Disulfide-bond-dependent antibody binding is partially recovered following reoxidation of reduced CD20. Antibody binding is unaffected by mutations of cysteines proposed to be in the intracellular domain of CD20. The affinities of intact rituximab and its Fab fragment to the isolated and purified CD20 are similar to the observed affinity of rituximab Fab for CD20 on the surface of B cells. However, the intact rituximab antibody shows much higher affinity for CD20 on B cells. This suggests that B cells display CD20 in such a way that allows for marked avidity effects to be observed, perhaps through cross-linking of CD20 monomers into lipid rafts, which limits receptor diffusion in the membrane. Such cross-linking may play a role in partitioning CD20 into lipid rafts and in enhancing antibody-dependent B-cell depletion activities of rituximab and other therapeutic anti-CD20 antibodies.