During the 1920s and 1930s geneticist L. C. Dunn of Columbia University cautioned Americans against endorsing eugenic policies and called attention to eugenicists' less than rigorous practices. Then, from the mid-1940s to early 1950s he attacked scientific racism and Nazi Rassenhygiene by co-authoring Heredity, Race and Society with Theodosius Dobzhansky and collaborating with members of UNESCO on their international campaign against racism. Even though shaking the foundations of scientific discrimination was Dunn's primary concern during the interwar and post-World War II (...) years, his campaigns had ancillary consequences for the discipline. He contributed to the professionalization of genetics during the 1920s and 1930s and sought respectability for human genetics in the 1940s and 1950s. My article aims to elucidate the activist scientist's role in undermining scientific discrimination by exploring aspects of Dunn's scientific work and political activism from the 1920s to 1950s. Definitions are provided for scientific discrimination and activist scientist. (shrink)