leukaemia. A consistently small proportion of the initially thouLght trivial, was always localised in the testes after 24 hours, and its clearance was slow. In animals retained for manv months for toxicological investigations an appreciable loss of testicular weight was observed consistently. After initial injection into six rats of 4mIn labelled leukaemia cells carrying a total dose of around 3-7 MBq a minute fraction (about 0-25'S,) appeared in the testes and had completely destroyed the seminiferous epithelium by 120 days, the weights of the testes in these rats being a third of those in matched controls. A parallel experiment with "4In-oxine labelled plasma produced destruction of the germ cell system by 70 days, with comparable percentage retention of the radioisotope in the testes. The radiation dose received by a testis was about 0-08 Gy/day immediately after injection and fell thereafter; this is only a fraction of the single external dose (
