We investigated the influence of TNF alpha and its muteins III, V and VI on the colony growth of normal and CML CFU-GM cells as well as on CFU-L clonogenic blasts from AML patients in cultures in vitro. The muteins were constructed using a synthetic cDNA fragment substituting nucleotides coding for the N-terminal amino acids in the chain of native TNF alpha. We observed that TNF alpha and mutein VI inhibited the growth of colonies formed by both types of CFU-GM and CFU-L cells in a greater degree than muteins III and V. In addition, the percentage of colony growth inhibition after the use of mutein VI was the greatest. These observations were the basis for the evaluation of interaction between 2-CdA and TNF alpha and between 2-CdA and mutein VI. We have confirmed that 2-CdA used together with mutein VI acts synergistically on CFU-GM and CFU-L cells, whereas 2-CdA and TNF alpha show the additive effect.