Mixed polyclonal cryoglobulinaemia was evidenced in a 49-year-old woman admitted to our hospital because of Proteus mirabilis sepsis associated with polyarthralgia and purpuric manifestations on the lower limbs. Cryoglobulins and circulating immune complexes decreased during the second week of illness and disappeared after recovery. CH50, C3 and properdin factor B, which were low during the early phase of the illness, returned to normal; C4 was normal throughout. The rapid clearance of cryoglobulins and immune complexes and the restoration of a normal complement profile might all be explained by the gradual elimination of P. mirabilis due to chemotherapeutic treatment.