1. Quadriceps muscle protein turnover was assessed in the post-absorptive state in six men immediately after the end of unilateral leg immobilization (37 ± 4 days) in a plaster cast after tibial fracture. A primed-constant intravenous infusion of L-[11-13C]leucine was administered over 7 h. Quadriceps needle biopsies, taken bilaterally at the end of the infusion, were analysed for muscle protein leucine enrichment with 13C. 2. Quadriceps muscle protein synthetic rate, calculated from the fractional incorporation of [13C]leucine into protein compared with the average enrichment of blood α-ketoisocaproate, was 0.046 ± 0.012%/h in the uninjured leg, but was only 0.034 ± 0.007%/h in the quadriceps of the previously fractured leg (P < 0.05, means ± SD). 3. Muscle RNA activity (i.e. protein synthetic rate per RNA) fell from 0.27 ± 0.08 μg of protein synthesized h-1 of RNA in the control leg to 0.14 ± 0.03 μg of protein synthesized h-1 μg-1 of RNA in the immobilized leg (P < 0.02). 4. Immobilization was associated with a significant atrophy of type I muscle fibres (mean diameter 69.5 ± 21 μm immobilized, 81.1 ± 18 μm control, P < 0.05), but no significant change occurred in type II fibre diameter. Mean quadriceps fibre volume calculated from the values for fibre diameter and percentage of each fibre type, was smaller in the injured leg by 10.6%, this value was near to the calculated difference in muscle thigh volume (calculated from thigh circumference and skin-fold thickness) which was less by 8.3%. 5. From estimated mean daily values for quadriceps protein synthetic rate (1.65 ± 0.44%/day in the control legs and 1.22 ± 0.28%/day in the injured legs) and change in fibre volume, mean daily muscle protein breakdown rates were calculated as 1.65%/day and 1.53%/day respectively, suggesting that muscle protein breakdown was not enhanced and may have fallen. 6. The results suggest a decrease in muscle protein turnover during limb immobilization in man, with the decrement in muscle mass being due mainly to a substantial (25%) depression of muscle protein synthesis.