A decrease of the natural killer activity (NKA) of C3HA splenocytes after transplantation of hepatoma 22a cells was connected to some extent with the suppressor action of macrophages and T-lymphocytes. Elimination of the macrophages from the splenocyte suspension led to the increase of NKA. Addition of the macrophages to the nonadherent cells resulted in the NKA inhibition to the level of nonfractionated splenocytes. T-cell elimination with anti-T-serum and a complement led to the increase of NKA as compared to NKA of the nonfractionated splenocytes.