We have studied the effects of sulindac and its main active metabolites on human bone marrow progenitors. Granulocytic-macrophagic colony forming units (CFU-GM) techniques were used for assessment. As in vitro inhibitory action of this drug or its metabolites on CFU-GM growth could not be demonstrated. These results refute the clinically presumed toxicity of sulindac on human bone marrow progenitors at pharmacological doses.