With the death of Harold Garnet (‘Mick’) Callan on 3 November 1993, the community of cell biologists lost one of the twentieth century's most profound and colourful students of chromosomes. During his 50-year scientific career the study of chromosomes and genes went from purely descriptive and morphological to deeply analytical and molecular. Steeped by training in the earlier tradition, Callan nevertheless contributed enormously to this revolution with his meticulous studies on the giant chromosomes of amphibians, all the while maintaining that he was a ‘mere cytologist’ on whom much of the molecular analysis was lost. Mick Callan and I were professional colleagues and close personal friends whose careers intersected at many points. We visited and worked in each other's laboratories, we published together, we generated a voluminous correspondence (much of it in the days when letters were handwritten), and our families enjoyed many good times together in Scotland and the USA. My most difficult task in writing this biography has been to extract from the vast amount of public and personal information in my possession those parts of Mick Callan's life and work that will be of chief interest to a broader audience. I have been helped in this by a 30 000-word autobiography written by him near the end of his life, covering the period from his birth in 1917 to the end of World War II in 1945. This account provides considerable insight into the factors that shaped his later professional career and is an engrossing account of the life of a boy in prewar England and a young man at Oxford and in the Royal Air Force (RAF) during the worst days of the war. Callan's autobiography has been deposited in the University library, St Andrews, Scotland.