L-2,4-Diamino[4-11C]butyric acid (DAB) was synthesized by an enzyme catalysed carrier added (0.1 micromol KCN) reaction of hydrogen [11C]cyanide with O-acetyl-L-serine followed by reduction. L-[11C]DAB was obtained with a radiochemical purity higher than 96% and with a decay corrected radiochemical yield of 30-40% within a 32 min reaction time. The enantiomeric excess was 98%. The uptake of L-[11C]DAB was investigated in multicellular aggregates of six different cell lines and animal tumour models. L-[11C]DAB is potentially useful for the assessment of pharmacokinetics of L-DAB in vivo for part of its evaluation as an antitumoural agent, although its use for diagnostic purposes seems limited.