Abstract: The present invention discloses a competitive lateral flow immunoassay for detection of recombinant bovine somatotropin (rbSt) in bovine milk or a milk product or extract thereof from lactating dairy cows treated/injected with rbSt. The rbSt (analyte) in milk or its derivatives, is added to a sample pad. The rbSt mouse or rat monoclonal antibody (primary antibody) is complexed to gold nano-particles and dried down on the conjugate pad. This complex (gold conjugated to monoclonal anti-rbSt) then flows to the test and control lines by capillary action through their respective channels, as a result of the osmolarity of the wicking pad. A solution of rbSt-BSA (Bovine Serum Albumin) is striped/bound to the test line. As the conjugate is rehydrated by the sample and flows down the strip, rbSt, if present in the test sample (milk), competes with rbS-BSA at the test line for binding to the gold labeled rbSt monoclonal primary antibody.