Upon completion of her habilitation, Kostial was appointed as a senior scientific associate and the head of the Department of Toxicology and Department of Biophysics at the Institute for Medical Research and Occupational Health in 1956, serving as a dual department head through 1964. Simultaneously she taught immunobiology and physiology in the Department of Natural Sciences at the University of Zagreb and between 1961 and 1962, returned to England studying on an International Atomic Energy Agency scholarship at the Radiobiological Research Unit of the Medical Research Council in Harwell. In 1964, she became head of the Department of Physiology of Mineral Metabolism and was promoted to scientific advisor serving in that capacity through 1989.[3]