We have tested serum samples from 24 patients with multifocal motor neuropathy (MMN) for reactivity to ganglioside GM1 and to Gal( β 1-3)GalNAc-bearing glycoproteins isolated from human peripheral nerve and from Campylobacter jejuni (Cj) serotype O:19. IgM anti-GM1 antibodies were detected by ELISA in 11 patients (45.8%) with MMN and in only one subject (4%) from the control group. Western blots showed positive reactivity of sera from 6 patients (25%) with MMN to several Gal( β 1-3)GalNAc-bearing glycoproteins from human peripheral nerve and from Cj O:19 isolates. Sera from three patients (12.5%) with MMN showed positively reactive bands with similar electrophoretic mobility in all isolates (60-62 kDa, 48-51 kDa, 42 kDa, and 38 kDa). All six patients showed positive reactivity to 48-52 kDa protein isolated from human peripheral nerve. Increased titer of IgG antibodies to 60-62 kDa protein isolated from Cj O:19 associated with Guillain-Barré syndrome was detected in three patients, and their serum showed also IgG positive reactivity to peripheral nerve antigen with the same electrophoretic mobility. One of these patients had a previous history of Cj infection which suggests the possibility that Cj may be also involved in the pathogenesis of MMN.