Microsomal contamination in nuclear preparations could represent one of the main sources of bias in the evaluation of the real metabolic capacity of the nuclear envelope. In this paper we present a quantitative study of the level of nuclear styrene monooxygenase enzymatic activity after artificially increasing the native microsomes to nuclei ratio. In one experimental conditions no significant elevation of the nuclear monooxygenase was observed. These data indicate that, if any microsomal contamination is present, it cannot account for more than 30% of the total enzymatic activity found in nuclear preparations.