1. The ‘flooding’ dose technique was used to measure rates of lymphocyte protein synthesis after infusion of [1-13C]leucine (20 atoms% enrichment, 4 g/70 kg body weight). Lymphocyte protein synthesis was measured in healthy subjects and in patients with metastatic colorectal cancer before and during infusion of recombinant interleukin-2. Rates of protein synthesis were compared with thymidine uptake in vitro and phenotypic analysis of lymphocytes.

2. The median rate of lymphocyte protein synthesis in four healthy subjects was 9% (range 7.2–11.4%/day) and in seven patients with colorectal cancer was 6.4% (range 4.2–8.2%/day). After recombinant interleukin-2 treatment the median rate of lymphocyte protein synthesis was 27.8% (range 25.2–33.7%/day).

3. The increased rates of lymphocyte protein synthesis in vivo, after recombinant interleukin-2 infusion, corresponded with increased rates of thymidine uptake and changes in the phenotypic expression of lymphocytes, but these were less consistent than the measured rates of protein synthesis.

4. It is concluded that lymphocyte activation is accompanied by a marked increase in lymphocytic protein synthesis which may have important implications for whole body protein metabolism. Furthermore, measurement of lymphocyte protein synthesis may provide a determination of lymphocyte activation in vivo.

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