Serum levels of C1q, C4, C3, C5, factor B, and properdin were measured in patients with Henoch-Schönlein purpura (HSP). In the cases of acute HSP, 9 of 23 (39%) had a low CH50, and 5 of 17 (30%) a low properdin; C1q, C4, and C3 levels were not depressed. In 10 cases with chronic nephritis following HSP, complement components were normal except for 2 with reduced C4 and one with low properdin. These findings confirm that complement activation occurs in HSP; the low serum levels of properdin in the acute group indicate that there is activation of the alternative pathway in these patients.