Abstract Allotypic determinants were detected on the Fab fragments of mouse IgG by guinea pig antisera prepared against polymerized Fab fragments of IgG from the C57BL and AKR strains. Different markers were detected by the two antisera. Indirect precipitation, with 125I-labeled Fab as ligand, was used for the assay. Each of the two markers was present on 20 to 25% of the Fab population. With one antiserum it was shown that this was due to restriction of the marker to one IgG subclass (IgG2a); it was inferred that a subclass restriction applies to the other antiserum as well. This, and the absence of markers on free light chains, suggest that the determinant(s) are present in the CH1 domain. Among various strains there is a close correlation between the possession of a Fab marker and the linkage group, based on Fc determinants, to which a strain belongs. With antiserum to C57BL Fab all strains in a given linkage group were the same with respect to the presence or absence of the Fab marker; strains carrying the Ig-1b allele were positive whereas all other strains tested were negative. Strains with a given allele also behaved similarly with respect to the anti-AKR Fab antiserum, except that this reagent divided the Ig-1a allelic group of strains into two subgroups.