Kai Adolf Jensen (1894-1971) became responsible for diagnosis of tuberculosis at Statens Serum Institut. He developed an improved medium for cultivation of mycobacteria allowing discrimination between Mycobacterium tuberculosis and M. bovis (Löwenstein-Jensen medium). He was also responsible for the Bacillus Calmette-Guérin vaccine production and supervised many young scientists’ doctoral theses on the aspects of tuberculosis. He became internationally recognized, and he revealed an important scientific fraud in the laboratory of the Austrian professor Ernst Löwenstein (1878-1950), who claimed that he could detect M. tuberculosis in blood cultures as a cause of a multitude of diseases. He was a pioneer in eradication of bovine tuberculosis in Denmark which became the first country in the world where this happened. Kai Adolf Jensen held the position as professor of general pathology from 1940 to 1965. In 1941, during the German occupation of Denmark in 1940-1945, he isolated a penicillin-producing Penicillium mold, which Leo Pharmaceutical Company used in the production of penicillin, and consequently Danish patients could be treated during the war. Soon thereafter, Danish penicillin was exported to a number of countries. Kai Adolf Jensen supervised many young scientists in the field of antibiotics and chemotherapy, and he had a unique impact on treatment of infections in Denmark.