Epithelial cells of the 15 .4 line, which were originally established from the adenomatous lung of a jaagsiekte case and which had been cultured in vitro for 22 generations, were injected subcutaneously into athymic nude mice. A slow-growing tumour which soon became cystic was established in each case. The cysts rapidly increased in size as a result of the accumulation of a slightly turbid secretion containing aggregates of tumour cells which rapidly refilled the cysts after the fluid had been withdrawn. Cultures were readily re-established from these cells and a chromosomal analysis proved that the tumour consisted of sheep cells. An epithelial cell lining, very similar to that found in the adenomatous lung alveoli of typical jaagsiekte, could be demonstrated histologically.