Fifty of 197 systemic lupus erythematosus (SLE) sera had 2 + positive C-reactive protein (CRP) determinations by precipitation in capillary tubes. All but 3 of these 50 sera had been taken at the time of infection whereas 80 of the 115 sera taken at the time of SLE activity without infection were negative for CRP. Thirty-two of the 35 CRP positive sera from patients with active lupus were obtained during intercurrent infection. CRP determinations in capillary tubes are clinically useful in distinguishing disease reactivation from intercurrent infection in SLE. These findings confirm a study using the same method for CRP determination. Discrepancy with another study may be due to the use of a different method.