Investigations carried out by Hueper (1955, 1957, 1958) and Hueper and Payne (1959) have contributed to our present knowledge of the cancerogenecity of metals and their role as occupational hazards, particularly in respect to lung cancer. Hueper considers arsenic, asbestos, chromium and nickel to be established metallurgical carcinogens. Besides epidemiological investigations, Hueper and co-workers administered various forma of chromium and nickel parenterally and powdered metallic nickel by inhalation in their studies of metal cancerigenesis. Cobalt was reported by Heath (1954, 1956) to be a potent carcinogen in rats with rhabdomyosarcomas amongst the neoplasms observed following intramuscular injection. Gillman and Ruckebauer (1962) confirmed these findings and stressed the high proportion of a "type of sarcoma that is clearly of muscle origin". They found that mice did not respond at all to this cancerigenic stimulus, a fact also noted by Heath (1959). Gilman (1962) showed in later work that nickel sulfide and oxide and the corresponding cobalt compounds were cancerigenic in rats on intramuscular injection. Again a very high percentage of the cobalt and nickel-induced tumours were rhabdomyosarcomas. However, the soluble nickel...