Using the tumorigenicity assay, we observed an activation of the mas oncogene during transfection of human acute myelocytic leukemia (CTV-2) DNA. Respective transfectants contained amplified mas sequences characterized by rearrangements in 5' and 3' noncoding regions that were transcribed into a 3.3 kb mas RNA species. Our data represent the second report on activation of the mas gene.