Using a previously developed in vitro model of sensitization of lymphoid cells against monolayers of syngeneic thyroid epithelial cells, the T or B nature of the population involved in the primary response has been investigated. For this purpose normal good responder CBA strain mice were used. T cell participation was clearly demonstrated in two ways: first, by the loss of primary sensitization after T cell depletion accomplished using several different techniques (including in vivo and in vitro anti-Thy-1.2 treatment, hydrocortisone and cyclophosphamide injections) and second, by the high response (measured by tritiated thymidine uptake) of T cell-enriched suspensions obtained either by nylon wool filtration or Ig-anti-Ig columns.