Twelve blood chemistry values, six haematological values and bodyweight were estimated for six-week-old specific-pathogen-free brown leghorns. Observations were based on 208 birds sampled from 11 groups. Subsequently the same values were estimated for specific pathogen free brown leghorns and broiler hybrids of differing ages. These estimates have been designated "comparison values" and were intended for reference use in clinical pathology. Results were compared with 14 "comparison value" estimated obtained previously for six-week-old commercial broiler hybrids. Haematological values varied with age and type of bird. Differences were observed between bird types in serum protein, serum cholesterol, uric acid, calcium and potassium levels and in weight. The significance of these differences is discussed.

1. Groups of 30 broilers from seven flocks were bled and weighed at the farm and a further sample of 10 broilers from each flock was transported from 8 to 16 km to the laboratory prior to bleeding and weighing. 2. One haematological and 11 biochemical analyses were made on each blood sample. 3. Although transportation did lead to significant differences in certain variables, the trends were consistent and sampling following transit is considered acceptable for clinical pathology studies.

Fourteen "comparison values" were determined for 6-week-old broilers. A total of 980 birds were sampled from 21 flocks to provide "comparison values" for body weight and blood characteristics. Production and management details are described and sources of variation in the "comparison values" within and between flocks are discussed. The presence of correlations between "comparison values" and between performance and "comparison values", both within and between flocks, is investigated.

A series of experiments is described in which E. coli septicaemia was produced in 6-week-old Brown Leghorn male chickens by air sac inoculation of a pathogenic strain of E. coli. Groups of birds were slaughtered for examination on days 1 to 5 post infection and haematological and biochemical examinations performed on blood samples collected just prior to slaughter. Changes in erythrocyte values and serum chemistry were observed and the clinical pathology is discussed. Consideration is given to the role of incidental haemodilution in the haematological changes observed.

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