The goals of the project are to identify and characterize the kinds of cells in mouse spleens which are active in immune responses. An improved method for physically removing macrophages has been developed and the requirement for macrophages for secondary immune responses established. A method for stimulating clones of T lymphocytes with hapten specificity has been developed and studies with these cells initiated. We have established that chronic allotype suppression is mediated by T lymphocytes. Data supporting the hypothesis that some T lymphocytes suppress the proliferation and differentiation of B lymphocytes has been obtained. The work accomplished thus far indicates that five distinct cells or cell functions involved in immune responses can be demonstrated and some progress has been made in determining the sequence of events and mode of interaction.