Previous research indicated that the production of peroxidases in Bjerkandera sp. strain BOS55 increased under N-sufficient conditions. In this study different organic N sources (amino acids, peptides, proteins) were tested as N supplements. Most N sources significantly enhanced the titres of peroxidases, which increased in parallel with growth. While peroxidase production by Bjerkandera sp. strain BOS55 was not repressed by the addition of sufficient or excess nitrogen, the occurrence of ligninolytic activity was a secondary metabolic event triggered by the cessation of growth. Among the factors studied (O-2, macronutrients, micronutrients, and vitamins), only those supplements with additional carbon or carbon and nitrogen increased the growth, indicating that the secondary metabolism was triggered by carbon limitation.