Haem (protohaem IX) analogues are toxic compounds and have been con-sidered for use as antibacterial agents, but the primary mechanism behind their toxicity has not been demonstrated. Using the haem protein catalase in the Gram-positive bacterium Enterococcus faecalisas an experimental system, we show that a variety of haem analogues can be taken up by bac-terial cells and incorporated into haem-dependent enzymes.