Recombinant antibodies represent a more sensitive and specific detection tool for immunoanalysis. The research carried out for this thesis describes the production of genetically-derived single chain antibody fragments to detect illicit drugs. A variety of novel recombinant antibody fragments against morphine-3-glucuronide, a metabolite of heroin has been produced. A monomeric, dimeric and enzyme-labelled scFv were characterised with respect to their binding abilities and cross reactivities. Monomeric scFv was successfully applied to a competitive ELISA format for the detection of morphine residues in saliva. The assay was used to positively identify morphine residues in the saliva of drug addicts attending a rehabilitation clinic. An inhibition assay to detect morphine in saliva was also developed using the BIAcore 3000 instrument, a surface plasmon resonance-based biosensor for detection of biomolecular interactions in ‘real-time’. A novel sol particle lateral flow immunoassay was generated using monomeric and dimeric scFvs for the detection of M3G. Recombinant antibody fragments against tetrahydrocannabinol were isolated from a naive human library by a process known as biopanning. Phage-displayed antibodies have been fully characterised with respect to their binding capabilities and were successfully applied to an immunoassay for the detection of marijuana residues in saliva.