B72.3 is a mouse hybridoma cell-line secreting an IgG1 antibody which recognises an epitope on a tumour-associated antigen, TAG-72. This high molecular weight mucin-like molecule is found on a variety of human neoplasms, including colon, breast and ovarian carcinomas. Chimaeric immunoglobulin genes with the B72.3 specificity have been constructed by joining the mouse variable regions from cDNA clones to human genomic constant regions using recombinant DNA techniques. The chimaeric heavy and light chain immunoglobulin genes were placed under the control of a strong viral promoter, and co-transfected into COS-1 cells. SDS-PAGE analysis of the 35S-labelled products demonstrated that the transiently expressed antibodies were correctly synthesised and assembled. The specific binding characteristics of the parent B72.3 antibody were retained by the chimaeric antibody in an antigen-based ELISA. This system gave sufficiently high transient expression of the chimaeric antibody molecules to allow rapid physical and immunological characterisation of the engineered gene products.