In a previous paper (Krawiec, Z., Biliński, T., Schüller, C. & Ruis, H., 2000, Acta Biochim. Polon. 47, 201-207) we have shown that catalase T holoenzyme is synthesized in the absence of oxygen after treatment of anaerobic yeast cultures with 0.3 M. NaCl, or during heat shock. This finding suggests that heme moiety of the enzyme can either be formed de novo in the absence of oxygen, or derives from the preexisting heme pool present in cells used as inoculum. The strain bearing hem1 mutation, resulting in inability to form delta-aminolevulinate (ALA), the first committed precursor of heme, was used in order to form heme-depleted cells used as inocula. The cultures were supplemented with ALA at the end of anaerobic growth prior the stress treatment. The appearance of active catalase T in the stressed cells strongly suggests that heme moiety of catalase T is formed in the absence of oxygen. This finding suggests the necessity to reconsider current opinions concerning mechanisms of heme synthesis and the role of heme as an oxygen sensor.