Extracellular lipase production and lipA expression were studied in A. calcoaceticus strains in relation to the nitrogen sources. Addition of casamino-acids or tryptone improved 2-3 times lipase yield as compared to ammonium, yeast extract or proteose-peptone. Further increase in lipase yield and enzyme stability were achieved when ammonium was also added. Differences in yields were not related to lipA transcription. Proteolytic activity was detected in culture broths simultaneous with lipase degradation. Post- transcriptional processes including enzyme protection, inactivation and secretion must be considered as important factors affecting lipase production.