The infectious plasmid OCT, which specifies a set of dissimilatory enzymes responsible for the degradation of n-octane, has been shown to be an aggregate of a noninfectious OCT plasmid and an infectious plasmid with sex factor activity. The infectious plasmid, which can be eliminated from the cells of Pseudomonas putida by mitomycin C treatment without loss of the OCT plasmid and vice versa, has been designated as factor K. The infectious plasmid (factor K) is not only responsible for the mobilization of OCT, but also mobilizes chromosomal genes at a frequency of 10(-2) to 10(-3) per donor cell. Whereas OCT is incompatible with another degradative plasmid, CAM, factor K appears to be compatible with it.