Much information is available concerning the influence of specific dietary factors on the cellular elements of the blood, but the practical question of whether the blood of cattle is affected during the rearing stage simply by the quantity of food fed has so far been neglected.The opportunity to investigate this point was provided by a long-term experiment in progress at this Institute, aimed at determining the effects of high and low planes of nutrition during the rearing stage on the subsequent growth and performance of dairy cattle. This experiment-of which only preliminary accounts have as yet been published (Crichton, 1953;Crichton & Aitken, 1954)-involves the use of monozygous twin calves, and for this reason it also enabled us to compare monozygous twins and unrelated animals in respect of their efficiency for haematological studies.Our investigations have shown that a high plane of nutrition can result in a distinctly improved blood picture, and also that the use of monozygous twins for this type of study leads to a marked increase in efficiency. MATERIALS AND METHODSMeasurements were made on the blood of eight pairs of monozygous twin heifer calves. Two pairs were on a high plane of nutrition, two pairs were on a low plane, while one member of each of the remaining four pairs was on the high plane and the other on the low plane. All the calves were cross-breds, but of mainly dairy type, and at the time of examination all were between 7 and 9 months of age.The high-and low-plane rations had been designed to correspond with 110 and 70 % respectively of the Ragsdale standard (for details see Crichton & Aitken, 1954). All the calves were reared on milk or milk substitutes up to 3 months of age, when they were weaned on to a diet of hay, turnips and concentrates, the differences in the rations fed being quantitative only. At 6 months of age the concentrate supplement was withdrawn from the lowplane animals, while calves on the high plane of nutrition continued to receive 2 lb./day.The haematological observations were conducted