l-2,4-Diamino[4- 1 1 C]butyric acid (DAB) was synthesized by an enzyme catalysed carrier added (0.1 μmol KCN) reaction of hydrogen [ 1 1 C]cyanide with O-acetyl-l-serine followed by reduction. l-[ 1 1 C]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-[ 1 1 C]DAB was investigated in multicellular aggregates of six different cell lines and animal tumour models. l-[ 1 1 C]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.