The synthetic terpolymer L-glutamic acid60-L-alanine30-L-tyrosine10 (GAT) stimulates GAT-specific suppressor T cells in nonresponder mice. Extracts from these T cells contain a GAT-specific soluble T cell suppressor factor (GAT-TsF) that inhibits development of GAT-specific plaque-forming cell (PFC) responses by spleen cells from nonresponder mice stimulated with GAT complexed to methylated bovine serum albumin (GAT-MBSA). These extracts also contain a factor that inhibits development of GAT-specific proliferative responses by GAT-MBSA-primed, nonresponder lymph node T cells. Experiments reported in this manuscript show that a hybrid T cell line, produced by fusion of the AKR thymoma, BW5147, with spleen cells that contain GAT-specific suppressor T cells, produces a constitutive GAT-specific suppresor factor that functionally and serologically resembles GAT-TsF extracted from T cells. More importantly, both GAT-specific PFC and T cell proliferative responses are inhibited by this factor.