Haemoglobin concentration (Hb), haematocrit (PCV) and mean corpuscular haemoglobin concentration (MCHC) were determined as indicators of oxygen-carrying capacity in 25 Simmental bulls during intensive fattening. Their possible relations to muscle growth were also investigated. The animals were the progeny of two sires. Blood samples were taken at 6, 9, 12 and 15 months of age. After slaughter, the right thoracic rib cut (7-9) was dissected into the longissimus dorsi muscle (LD), other muscles (OM) and total muscles (TM) (LD+OM). There was a significant increase in the mean PCV and Hb values during fattening. The PCV and Hb values were significantly correlated with the tissue share of OM or TM only at 12 and 15 months of age. LD muscle share was not significantly correlated with any of the parameters at any fattening time. It would appear from these results that muscle growth proceeds in two phases. The first is until the age of 12 months and the other from then to maturity.