ABSTRACT:Complement components C1q and C4 of classic pathway; C3d, a breakdown product of C3, and factor B of alternate pathway: and C3, a component both of classic and alternate pathways, were studied in 35 patients, comprising 18 type I (Lepra) and 17 type II (ENL) reactions. There was a significant decrease in C3 and factor B with a concomitant rise of C3d during ENL. These changes indicate their preeminent role in immunogenesis of type II (ENL) reaction. The changes in the classic pathway components, on the other hand, were insignificant, apparently suggesting its limited involvement in ENL. Furthermore, reversion of factor B and C3d after subsidence of reaction is intriguing and may indicate that they are not substantially affected even with contemporary treatment. Complement components, of both classic and alternate pathways, showed no significant alterations either during type I (Lepra) reaction or after its amelioration.