Using rosetting methods, we have purified rabbit B cells and studied their interactions with T cells purified by passage over an anti-immunoglobulin-coated Degalan beads column. B cells enhance the response of T cells to concanavalin A (Con A) and phytohaemagglutinin. In regulation of the response to Con A, an adherent cell is a third participating cell. B-cell preparation contain a minority of cells that can respond to T mitogens with the help of non-proliferating T cells, but the proportion of these responding cells is small, and the involvement of the T-cell impurity cannot be excluded.