Evidence that thymocytes require at least two distinct signals to proliferate.

Optimal receptivity of thymocytes to the mitogenic stimulus in macrophage culture fluid (MCF) is limited to the time in culture when these cells are spontaneously proliferating. By providing a brief, nonmitogenic lectin pulse, nonproliferating cells can be "activated" and their ability to respond to MCF totally restored. This procedure provides a model system for evaluating thymocyte proliferation and, in particular, the mechanism of action of MCF.