A recombinant chimeric Fab (rcFab), with Fv derived from the monoclonal A5B7 antibody to carcinoembryonic antigen (CEA), and with human CHI and Ck was cloned into pUC 19 and expressed in Escherichia coli. rcFab (10 to 12 mg per litre) was produced in bacterial culture fluid, and functional purified rcFab was isolated by affinity chromatography (using antibody to human Ck) and size-exclusion gel filtration. The re Fab did not show reduced affinity for CEA, and reacted with human colorec-tal tumours showing a typical anti-CEA pattern by immunocyto-chemistry; it was also stable after iodination. Biodistribution studies in nude mice bearing human tumour xenografts showed no toxicity and good turnout- localization. Therapeutic ratios at early time points were better than those obtained with whole murine antibody. The results demonstrate that bacterial I y produced anti-CEA Fab is of use for tumour targeting. © Wiley-Liss, Inc.