Individual murine B cells express a wide range of Ia densities on the plasma membrane. Here we demonstrate that a dramatic increase in B-cell Ia could be induced by overnight exposure to an uncharacterized lymphokine (LK). Membrane I-A and I-E molecules were both increased after LK treatment, whereas membrane IgM remained unchanged. Two subpopulations of B cells were identified, based on their requirements for expressing maximal Ia; one subpopulation required only LK, the other required both LK and T cells in the overnight culture. Functional changes accompanied the Ia increase. The functional capacity to present antigens to T cells was lacking in normal resting B cells, but was acquired following LK treatment. We suggest that the LK-treated B cell has achieved a new differentiation state, one of preparation for interaction with T cells. We term this state the "poised" B cell, and propose that B cells in the poised state may significantly contribute to T-cell activation as antigen-presenting cells. Moreover, poised B cells may themselves find an advantage over normal B cells in successfully acquiring T-cell help.