I propose to study the ontogeny of B lymphocyte subpopulations responding to thymus dependent and thymus independent antigens in vitro. Recently we and others have published evidence supporting the existence of such subpopulations among IgM antibody-forming cell precursors. We now have preliminary evidence for similar subpopulations among IgG resting memory precursors. The placement of these subpopulations of thymus dependent and thymus independent B cells in the developmental pathway(s) leading to humoral immunity and memory is unknown. Using the technique of precursor frequiency analysis, selective suicide and B cell cloning, I propose to determine a) whether they constitute a continuous line of development or whether they arise from separate precursors, b) the relative rate at which these populations arise in adult tissues, c) the rate at which they arise from neonatal immunosuppressed tissues, d) whether antisera can be raised against these subpopulations to be used as markers of differentiation. The answers to these questions may help to clarify the role these subpopulations play. These subpopulations as currently defined may or may not reflect true differentiation stages but rather expressions of environmentally induced modulations; nevertheless, they offer one means of gaining an insight into functionally differentiated members of the B lymphocyte network and the importance of antigenic structure in directing the nature of the responding cell.