A modification of double-antibody sandwich ELISA (enzyme-linked immunosorbent assay) has been applied successfully to the problem of the detection of very low amounts of pig meat (1–3%) in beef and manufactured beef products made with rusk and other additives. The assay uses a monospec fic antiserum bound to the solid plastic support to sequester the porcine serum albumin (PSA) from amongst the many proteins co-extracted from the complex sample extracts. Further immuno-recognition was made with a second antibody, also specific for PSA, which was detected by a conjugated enzyme anti-IgG that interacted only with the second anti-PSA antibodies. Subsequent enzymic conversion of substrate gave clear visual differences between beef and beef containing 0·5% of pig meat and beef products with 1% of the lean replaced by lean pork compared with reference samples.