The relationship between immune complex, DNA binding and complement levels and disease activity was studied in twenty-seven patients with systemic lupus erythematosus. Detailed longitudinal studies revealed a good correlation between immune complex levels and disease activity. In addition, a good correlation was obtained, in general, between immune complex and DNA binding values, and an inverse correlation between these values and the haemolytic activity of C1, C2 and C4. CH50 and C3 measurements, on the other hand, were found to be less consistent at reflecting short term fluctuations in disease activity. Statistical analysis of IC, DNA binding and complement values revealed a higher incidence and mean value of immune complexex in active disease compared with inactive disease. These high levels of immune complexes were associated with elevated values of DNA binding and low values of CH50 and C3. The highest values of immune complexes were found in samples with concomitantly abnormal values of DNA binding, CH50 and C3. It is concluded that the measurement of immune complexes in SLE patients is a useful procedure, not only in aiding diagnosis, but also in monitoring disease activity and response to therapy. The results also indicate that serial studies of immune complexes, DNA binding and complement can be of value in the long term management of individual patients.