Benzathine penicillin, procaine penicillin, and mixtures of both are opaque preparations for intramuscular injection. Visualization of aspirated blood, the fail-safe for avoiding intravascular injection, is predictably problematic. A spectrum of injuries, sometimes permanent, to the gluteal region, distal extremities, perineum, and spinal cord probably results from inadvertent intra-arterial injection, possibly due to vascular occlusion by large crystals of the penicillin salt(s). Since the arterial injurie disproportionately affect infants, use of these preparations in that age group should be minimized. The corresponding intravenous injection probably is Hoigne's syndrome, a procaine-induced central nervous system syndrome. Though usually brief and transient complications may be profound, including coma, convulsions, and death. Careful attention to details of anatomy and manufacturer's recommendations is critical.