The first step in caprolactam polymerization is an addition polymerization. The authors have studied the initiative influence of various substances on the polymerization and obtained the following results. The initiators of the free radical type, such as benzoyl peroxide and hydrogen peroxide together with their redox systems, all exerted no effect on caprolactam. While carboxylic acids gave only weak initiative effect in absence of water and amines gave no action, the mixtures of acids and amines and of acids and water initiated the polymerization rapidly.