Line cells specific toward FYAK or GA responded similarly to their corresponding copolymer. Copolymer-specific line cells were collected on day 2 of re-stimulation, washed and cultured at 1.25 × 105/well, along with the same number of irradiated syngeneic APC, and with the corresponding copolymer at different concentrations, as indicated. The proliferation level was determined by 3H-thymidine incorporation, measured after a pulse of 0.5 μCi/well during the last 16 hr of the 90 hr total incubation. The data are expressed as delta CPM+/- SEM.