The existence of precursors for TI and TD alpha 1 leads to 3 dextran antigens in BALB/c mice was demonstrated. A T-dependent dextran antigen was prepared by coupling dextran B1355S to hemocyanin and subsequent digestion with dextranase. The PFC response of BALB/c mice primed with hemocyanin to dextran-hemocyanin was found to be 8 times higher than in unprimed animals. The splenic focus assay was adapted for the analysis of precursors responding to T-dependent and T-independent dextran antigens. Pretreatment of recipients with anti-thymocyte serum abolished the response in fragment cultures to dextran-hemocyanin but did not affect the response to dextran B1355S. The frequencies of precursors in the adult BALB/c mouse responding to dextran and dextran-hemocyanin were determined by limiting dilution analysis. The frequency of T-dependent precursors was found to be almost 3 times greater than the frequency of T-independent precursors.