The aim of the present work was to analyze the frequencies of a genetically defined variable (V) region marker in the B cell subset sensitive to T cell help. To this end we used an alloreactive T cell line that has the property of inducing B cells of the appropriate haplotype to exponential growth and polyclonal antibody synthesis. The frequency obtained with this helper line was also directly compared to that obtained with lipopolysaccharide (LPS). We found that in normal BALB/c mice the frequency of M460-positive clonotypes was respectively, 1/100 and 1/1,000 among the T helper- and LPS-sensitive B cell subsets. In mice immunized with antiidiotype coupled to a thymus-dependent antigen, the differences in the numbers of idiotype-positive precursors were even more accentuated, i.e. 1/20 in the B cell subset triggered by T helper cells and 1/800 in those cells responsive to LPS. The frequencies of the M460 determinant in mice immunized with anti-idiotypes coupled to thymus-independent antigens were not significantly different, in either B cell subset, from those obtained with spleen cells of normal nonimmunized animals. Taken as a whole, our results imply that the V gene repertoire revealed by LPS includes precursor distribution, as this distribution occurs during the early stage of B cell development (potential repertoire), while the repertoire revealed by T helper cells includes the V region distribution of those clones that are selected in the periphery of the functional immune system.