C2 hemolytic activity has been measured in serum and C.S.F. of 67 multiple sclerosis (M.S.) patients classified in three evolutionary groups according to the index of progression of the disease and tested in HLA‐A, B, C, DR and Bf. The median level of serum C2 is normal whether it is of the evolution or the HLA type. The average level of C.S.F. C2 is the same in the evolutionary groups and is low in patients bearing B18 allele or B18, DR2 haplotype. It seems that the median level of C.S.F. C2 is linked to the HLA types associated to the susceptibility gene(s) of the diseas