In view of the fundamental importance of the reaction of penicillins with amino groups of proteins to the penicillin allergy, the aminolysis of benzylpenicillin by various amines was kinetically investigated. The formation rate constants, kamide, of benzylpenicilloylamides were determined at 35 degrees, 45 degrees and 60 degrees (mu equals 0.5), and found to obey the general rate law: kamide equals k1[amine] + k2[amine H+] [amine] + k3[amine]2 + k4[amine]aoh. All of the amines exhibited the unassisted nucleophilic rate constant, k1. The relative importance of the other kinetic terms depends on the basicity and the chemical structure of amines. The reaction mechanism of penicillin aminolysis was discussed. Bronsted relations for k1, k2 and k3, except for hydrazines, were satisfactory.