By means of the agglutination by the peanut agglutinin(PNA),a population of PNA~+ lymphocytes which bear peanut agglutinin receptors,were separated from the spleen cells of mice.The relative number,functional activity and inducibility of the PNA~+ lymphocytes were examined in the spleen cells of normal and EAC tumor-bea- ring mice.The results indicated that the PNA~+cells of mouse spleen exhibited sup- pressive effect on cellular immunity judging from an assessment of modified local xenogeneic graft versus host reaction(GvHR)in guinea pigs.The GvHR induced by PNA~+-cell-depleted spleen cells(PNA~-)from tumor-bearing mice appeared to be signi- ficantly stronger than that induced by unseparated spleen cells from the same donors. The demonstrations of cytolysis of predominant PNA~+ cells treated with anti-Thy 1 serum and abolishment of suppressive activity of the PNA~+ cells thus pretreated con- firmed the impression that the PNA~+ lymphocytes essentially belong to T cell lineage enriching in T suppressors(Ts).Using the fluorescein labelled PNA(PNA-FITC)tracing technique,it has been observed that the relative number of splenic PNA~+ cells in the tumor bearing mice increased concomitantly with the tumor progression.It was also noted that when normal spleen cells were exposed to cell-free ascitic fluid from EAC- bearing mice for appropriate incubation period,a higher percentage of PNA~+ cells was obtained after incubation,and thus induction of PNA~+cells was shown.The nature of the inducing factor(s)as well as the mechanism of induction remain to be clarified.