In March 2004, the Los Angeles County Department of Health Services (LACDHS) was notified that a large nonprofit clinic serving the gay and lesbian community in Los Angeles used a nonrecommended preparation of penicillin to treat syphilis patients during January 1999-March 2004. The clinic had inadvertently used Bicillin C-R, a mixture of 1.2 million units (MU) benzathine penicillin G (BPG) and 1.2 MU procaine penicillin G, rather than Bicillin L-A, a preparation that contains the 2.4 MU BPG per dose recommended by CDC. Bicillin L-A is recommended for treating syphilis and upper respiratory tract infections caused by susceptible streptococci. Bicillin C-R is indicated for streptococcal infections of the skin and respiratory tract; however, its efficacy in treating syphilis is unknown. The inadvertent use of Bicillin C-R, which contains only half the recommended dose of BPG for syphilis, was discovered after a patient treated for syphilis read the product insert, which stated that the medication was not indicated for treatment of syphilis. Review of clinic pharmacy records revealed that it received a shipment of Bicillin C-R in lieu of an unfilled order for Bicillin L-A in late 1998 and that the pharmacy subsequently ordered Bicillin C-R until March 2004. The clinic used Bicillin C-R as its exclusive formulation of injectable penicillin during January 1999-March 2004. This report summarizes the investigation of the misuse of Bicillin C-R at the Los Angeles clinic, which represents the largest occurrence of inadvertent treatment with Bicillin C-R to date. The investigation led to discussions among CDC, the Food and Drug Administration (FDA), and King Pharmaceuticals, Inc. (Bristol, Tennessee), whose Monarch Pharmaceuticals subsidiary markets Bicillin products. As a result, King Pharmaceuticals agreed to institute packaging and labeling changes to Bicillin products to prevent inadvertent treatment of syphilis with Bicillin C-R.