Erythrocytes from 30 patients suffering from systemic lupus erythematosus (SLE) were tested for CR1 activity by an immune adherence haemagglutination technique. Defective CR1 activity (CR1D) was found in 11 (37%) of the patients on initial testing. On repeat testings, however, CR1 activity often varied from time to time and was shown to be inversely related to serum anti-DNA binding and apparent complement activation in vivo. Two of the 19 patients with normal CR1 activity acquired CR1D during the study. One patient with previously defective CR1 attained normal activity in the course of the study. The increased occurrence of CR1D in patients with SLE is largely or wholly acquired rather than genetically determined.