Concentrations of IgG allotypes G1m(1), G1m(3) and G3m(11) in neutral pH eluates from discrete plaques of multiple sclerosis (MS) brain and from white matter of control brain were determined to obtain information about distribution of B cell clones among MS lesions. Within each MS brain a predominant nominal IgG1 allotype was distributed rather homogeneously in all plaques while quantitatively minor allotypes showed some fluctuation. Latent IgG1 allotypes were detected (7-12% of the corresponding nominal allotype level) in some tissue eluates from both MS and control brains, which were homozygous for either G1m(1) or G1m(3). By contrast, the expression of a latent IgG3 allotype, namely G3m(11), was apparently MS restricted. Large amounts of latent allotypes were detected only in recent plaques with lymphoid cells whereas the distribution of total plaque associated IgGs did not correlate with the presence of lymphoid cells. Latent allotypes in recent MS lesions may mark a transient immunological activity which coincides with the infiltration of lymphoid cells and precedes the appearance in these plaques of oligoclonal IgGs, the distribution of which may parallel that of the predominant nominal allotypes.