Mammalian cytochrome P450 enzymes are of special interest as biocatalysts for fine chemical and drug metabolite synthesis. In this study, the potential of different recombinant microorganisms expressing rat and human cyp1a1 genes is evaluated for such applications. The maximum specific activity for 7-ethoxyresorufin O-deethylation and gene expression levels were used as parameters to judge biocatalyst performance. Under comparable conditions, E. coli is shown to be superior over the use of S. cerevisiae and P. putida as hosts for biocatalysis. Of all tested E. coli strains, E. coli DH5α and E. coli JM101 harboring rat CYP1A1 showed the highest activities (0.43 and 0.42 U g⁻¹(CDW), respectively). Detection of active CYP1A1 in cell-free E. coli extracts was found to be difficult and only for E. coli DH5α, expression levels could be determined (41 nmol g⁻¹(CDW)). The presented results show that efficient expression of mammalian cyp1a1 genes in recombinant microorganisms is troublesome and host-dependent and that enhancing expression levels is crucial in order to obtain more efficient biocatalysts. Specific activities currently obtained are not sufficient yet for fine chemical production, but are sufficient for preparative-scale drug metabolite synthesis.