Cytochrome P450 1B1 (CYP1B1) is a human extrahepatic P450 that activates procarinogens, metabolizes 17beta-estradiol, and may well have a role in the pathogenesis of some forms of cancer. Besides rare deleterious mutations reported for the CYP1B1 gene, six single-nucleotide polymorphisms have been reported, of which four cause amino acid exchanges. We have expressed two of the common CYP1B1 alleles in yeast cells and mammalian COS-1 cells in order to functionally characterize the alleles with respect to kinetic properties and protein stability. The CYP1B1.2 variant contains the two linked amino acid substitutions R48G and A119S compared to CYP1B1.1. The kinetic parameters of two structurally unrelated CYP1B1 substrates for the two variants were examined. No kinetic differences were seen of 17beta-estradiol hydroxylation activities between the two CYP1B1 variants and an only minor increase in the apparent Km for ethoxyresorufin was observed for CYP1B1.2. It therefore appears that they have very similar catalytic properties and the substitutions do not appear to alter CYP1B1 catalytic function. The two CYP1B1 variants were similarly stable when expressed in mammalian COS-1 cells, indicating that the substitutions have no effect on protein folding or stability. The combined results indicate that these two CYP1B1 variants show very similar properties with respect to catalytic activities and protein stability and do not alter CYP1B1 function.