1. Seven sets of monozygous twins were kept under the same open-grazing conditions for one year. Milks were analysed for fat, solids-not-fat, lactose and total protein contents, and butterfats were isolated at fortnightly intervals for estimation of iodine value, refractive index, softening point, saponification value, Reichert value, carotene content and vitamin A content. The results are presented both graphically and statistically.
2. Consistent differences in body weights of the cows of a twin set did not render the twin set unsuitable for use in experimental work on causes of changes in milk yield, fat yield, milk composition or butterfat characteristics.
3. The advantages of monozygous twins over unrelated cows was shown: (a) by the close general correspondence of the within-twin-set results as compared with the between-twin-set results throughout the period of the trial; (b) by the similarity in the forms of the curves for the different sets of twins; (c) by the marked similarity of the reaction of the cows of a twin set to change in environmental conditions—this applied particularly for butterfat characteristics such as iodine value and refractive index; (d) by the ‘twin efficiency values’ for butterfat properties computed from the whole-lactation weighted means, viz. iodine value, 7·8; saponification value, 3·4; Reichert value, 5·5; carotene content, 7·8; vitamin A content, 1·7; and vitamin A potency, 2·5.
4. There were wide differences in the degree of approach to ‘identicality’ with different sets of twins. The cows in two of the sets showed a consistently close within-twin-set relationship for all characteristics studied.