Abstract: Novel monoclonal antibodies capable of distinguishing between cyclosporins, e.g. Cyclosporine, and metabolites, e.g. Cyclosporins 17 and 18, are produced, e.g. starting from novel cyclosporins having an activated coupling group, e.g. activated carboxy group, e.g. (i) [(O-succinimidooxysuccinyl)-Thr].sup.2 -Cyclosporine and (ii) [(N-.epsilon.-succinimidooxysuccinyl)-(D)Lys].sup.8 -Cyclosporine. Cyclosporin starting materials required for the production of cyclosporins of type (ii), e.g. [(D)Lys].sup.8 -Cyclosporine are also new and additionally have utility in the preparation of novel labelled cyclosporin derivatives, as well as antibodies and antisera generally. Also claimed are novel antigenic conjugates and hybridoma cell lines used in the production of antibodies and antisera as aforesaid as well as assay kits comprising novel antisera, antibodies and/or labelled cyclosporins as aforesaid.