The following 25 papers represent studies on a number of different aspects of disease in the domesticated fowl. As the majority are on the subject of neoplasia, they will be summarised first. They are divisible into studies from the aspects of comparative pathology (Nos. 1, 2, 3, 6, 13); the classification of the leucoses (Nos. 9, 12, 18); the tumour viruses (Nos. 7, 8); chemical induction experiments (Nos. 10, 11); enzyme histochemistry (No.5.); and endocrine studies (Nos. 14, 15, 16, 17). In addition, there are five papers of joint authorship on miscellaneous subjects. The earlier papers are based on work done while the author was in charge of the Poultry Diseases Department at the (then) Royal (Dick) Veterinary College, whilst the later papers are the result of work undertaken subsequent to appointment as Research Officer in the British Empire Cancer Campaign Unit at the A.R.C. Poultry Research Centre, Edinburgh. The paper on Bangkok Haemorrhagic Disease (No.20) is based on material collected while working in an advisory capacity to the Government of Thailand, for the Food and Agricultural Organisation of the United Nations.