Mutations in IGF-II (insulin-like growth factor-II) and RYR1 (ryanodine receptor-1) increase lean meat content in pigs but with different effects on meat quality. Differences in biochemical and histochemical muscle fibre characteristics between the two mutations can thus be suspected. Therefore, the aim of this study was to investigate the effect of the IGF-II mutation (Apat vs. Gpat) on biochemical and histochemical muscle fibre characteristics in relation to the RYR1 genotype (Nn vs. NN). A prenatal effect of the IGF-II mutation could not be excluded but the increased leanness in both mutations was clearly related to postnatal muscle hypertrophy due to an increase in muscle fibre diameter and a higher proliferative capacity in animals carrying the IGF-II mutation. No effect of the IGF-II or RYR1 mutation was found on fibre type composition and metabolic enzyme activities. Interactions between IGF-II and RYR1 genotypes suggest that the mechanism involved in increased leanness due to the impaired RYR1 receptor might influence the underlying mechanisms of the IGF-II mutation. (C) 2007 Elsevier Ltd. All rights reserved.