A microlymphocytotoxicity test determined serologically the frequency of HLA antigens in 32 patients with Guillain-Barre syndrome and in 234 healthy control subjects. The results demonstrated significantly increased frequencies of A3 and B8. The relative risk was estimated to be 9.6 and 4.6 for the A3 and B8 antigens, respectively. Study of the gametic association revealed weak positive linkage disequilibrium and biological association. The results are discussed, and it is concluded that the aberrant genetic make-up of the patients makes them more susceptible to develop the syndrome after exposure to the environmental factor(s).