Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry is employed to detect the presence of the bacteriological sex factor, a trait that allows some bacteria to transfer genetic information to others by means of conjugation. The principal advantage of the method is its speed. Twenty different strains of E. coli were analyzed and the results were consistent with previously known genetic information. The sex factor could be transferred from one strain to another and the outcome verified by mass spectrometry.