A 48 year old man with an onset of macules and papules, ulcers and eschars on the upper and lower extremities, hands, genitals, face, as well as nasal and oral mucosae. The rash was accompanied by severe malaise, fever, ahralgia, paraesthesia, and microhematuria. Cryoglobulins and high titers of circulating immune complexes were noted. Steroid treatment was instituted together with plasmapheresis with good results. This is most likely secondary to: