The flock at the Poultry Research Centre (Greenwood and Carr, 1951; Campbell, 1963a) Is considered to be free from leucosis, and since no case of leucosis has arisen in birds injected with the osteopetrosis preparations, the assumption has been made that the virus 'strain* is, in fact, free from leucosis producing agents. Since the gross pathology of the disease has been adequately reported, the experiments reported in this work have been directed towards the characterisation of the virus, rather than the effect on the infected cells. On these assumptions, a series of experiments has been carried out, which aimed at showing 1. that osteopetrosis is, indeed, transmissible by a virus; 2. the character of the virus; 3. the behaviour of the virus in vitro, with a view to developing an assay method; 4. the life cycle; 5. the relationship, if any, of osteopetrosis with the leucosis complex and the transmissible R.P.L. 12 lymphoid tumour.