Sir.—The original article by Silber and D'Angelo,1 as well as the letters and replies,2-5 describes the clinical manifestations of a nonallergic procaine reaction. No clear explanation of the mechanism of the reaction is obvious; however, the elevated blood level of procaine has been implicated, 6 and the symptoms match those of intravenous procaine administration.7
It seems unlikely that true intramuscular injection would lead to rapid elevation of blood levels of one of the contents. Intra-arterial injection of penicillin G benzathine8 results in a spectrum of gangrenous and hemologic consequences in the area supplied by the arterial branches.9 The fact that the fail-safe feature—the inability to see blood in the syringe or tubex—failed has also been argued.10 Penicillin G benzathine preparations are opaque, easily concealing blood in the syringe and obscuring the only indication of an intravascular injection site. The most plausible explanation for