The disagreement between various immunoassays for the tumour marker carcinoembryonic antigen (CEA) is well known, and the discrepancies are explained by the co-measurement of the structurally related non-specific cross-reacting antigen 2 (NCA-2). Until now, this hypothesis has only been proven indirectly, as no NCA-2 specific assay has been available. This thesis describes the production of a high affinity single-chain antibody fragment (scFv) raised by using phage display technology. The scFv was used to construct an immunometric assay that specifically recognises NCA-2. A phagemid library was constructed from splenic mRNA obtained from a mouse immunised with NCA-2 purified from human meconium. Following phage rescue and three rounds of selection on solid phase coated with NCA-2, several clones, which displayed scFvs with high specificity and affinity for NCA-2, were isolated. These were sub-cloned into an expression vector for high-level expression of soluble scFv. Based on BIAcore analysis, a scFv with low koff and a KD of 10-10 mol/l was selected for europium labelling and employed as the tracer molecule for the NCA-2 - specific immunofluorometric assay. This novel assay was applied for the quantification of NCA-2 in sera from 35 patients with elevated CEA levels. The measured NCA-2 correlated fairly well (R2=0.82) with the values obtained by indirect methods based on the differences between serum values measured by NCA-2 cross-reactive and CEA specific immunoassays.