A highly sensitive immunoassay system involving antibody-coated tubes and liposome-entrapped dye.

In this colorimetric immunoassay for digoxin, large, unilamellar phospholipid vesicles approximately 0.2 micron in diameter are loaded with high concentrations of Sulforhodamine B. Digoxigenin coupled to phosphatidylethanolamine, incorporated into the lipid formulation, confers immunological specificity. The liposomes are then used as tracers in simple competitive-binding immunoassays with antibody-coated tubes. Results are amplified by 10(3) to 10(4) of what could be achieved with one label group attached to each hapten, so that the results can be read spectrophotometrically. The stability of the liposomes is excellent. The method should be applicable to measuring a wide variety of analytes.