Trichostatin A (TSA) induced the normal and flat phenotype of sis-transformed NIH3T3 cells at quite a low concentration of 1 ng/ml. Although morphological changes were found in other oncogene-transformed cells, they were not the same as those seen for the sis-transformed cells. Almost complete reversion into the flat phenotype was seen at 6 h after administration of the compound, suggesting that the morphological change was caused not merely by selection of TSA-resistant cells of the flat phenotype. The effect of TSA was reversible when the cell culture was incubated after its removal. Synthesis of sis-mRNA did not decrease with the treatment of TSA at a concentration sufficient to reverse the transformed morphology. Cycloheximide abolished the activity of TSA, showing that TSA required new protein synthesis to express its activity.