The Jarisch-Herxheimer reaction was first described by Jarisch (1895) and the concept was elaborated by Herxheimer in 1902 (Stokes, Beerman, and Ingraham, 1944). The reaction was first observed after mercury inunctions; it can occur, mildly, after bismuth; it was a frequent sequel to the first injection of an organic arsenical, and it occurs in about 50 per cent. of cases of early syphilis after the first dose of penicillin. The reaction usually consists of a single episode starting some 2 to 12 hours after the beginning of therapy, and in cases of early syphilis a well marked general reaction presents with malaise, headache, flushing and sweating together with fever which may reach 104°F. Often there is an accompanying local response; a chancre may become swollen and painful; there may be the transient first appearance of a secondary rash, or a pre-existing rash may become more dense and florid. Most general reactions have passed off by the following day. Repeated systemic reactions have been evoked experimentally by giving successive small doses of penicillin (Farmer, 1948; Gudjonsson and Skog, 1968), but with doses of the usual therapeutic range a second reaction is undoubtedly a rare event. The purpose of this report is to describe such a case.