Without knowledge of potential protein growth, nutrient requirements of pigs cannot be accurately determined. Daily protein retention [Pr] was estimated with 45 crossbred pigs serially slaughtered between 20 and 200 kg body weight. An understanding of growth to maturity is vital for the adequate nutrition of pigs grown to slaughter at heavier weights and for breeding sows, but the determined function: Pr(kg) - 0.125 [±0.009] -0.0002 [±0.0001] mean live weight, having no significant slope, was an inadequate descriptor of instantaneous Pr for pigs of more than 110 kg. Allometry was therefore used to predict protein weight at any given body weight and the Gompertz function to express body weight changes with time and derive values for weight at maturity. Predicted values for Pr attained maxima at 75 kg body weight of 130 g for entire males. 120 g for females and 105 g for castrates. Between 45 and 110 kg body weight Pr was within 10 g of the maximum rate (Figure 1); mature body weights were estimated to be 240, 215 and 225 kg. Allometric expressions for dissected carcass and chemical components as a function of empty body weight are given in Table 1. Figure 2 compares currently predicted potential rates of protein retention with those of Carr et al.  and Thorbek (19751. It is evident that weight and age at maturity are crucial to the quantification of protein growth; such measurements are prerequisite to the provision of a quantitative description of improved pig genotypes.