A method for screening of ketone biotransformations was developed and applied to the identification of Baeyer-Villiger monooxygenase (BVMO) activity. The method was based on the formation of a purple coloured product on reaction between an enolizable ketone and 3,5-dinitrobenzoic acid in an alkaline solution. Absorbance of the colour decreased with the size of the cycloketone ring. Stoichiometric ratio between cycloketone and 3,5-dinitrobenzoic acid was 1:1 at maximum absorbance. The method was applied for monitoring the consumption of cyclohexanone by bacteria under aerobic conditions, and was found to be potentially useful for both screening assays and quantitative measurements of BVMO activity. Compared to other existing methods, this method is faster, cheaper and amenable for whole cell assays.