The biological efficiency of meat production (E) is defined as the weight of carcass produced per 100 units of digestible organic matter (DOM) consumed. The factors affecting this relationship, for ewes and lambs, are discussed and the results of experiments are used to demonstrate the effect of number of lambs per year and size of ewe on the value of E. Calculations are made to show the effect on E of other factors such as the growth rate of the lambs and the weight at which the lambs are slaughtered. The general conclusions are that the highest values for E will be obtained from small breeds of ewes producing large litters and crossed with a large breed of ram, leading to a high growth rate and final size in the lamb. A relatively small increase in litter size in the small breeds of ewe (i.e. from one to two lambs) may result in a level of efficiency as high as that achieved by a larger breed with a large litter size (i.e. three to four lambs) without having to resort to techniques such as the artificial rearing of lambs.