A method of measuring the amount of cyclosporin in a sample suspected of containing cyclosporin is disclosed. A method of inactivating interfering cross-reactive material in an assay for measuring the amount of cyclosporin in a sample suspected of containing cyclosporin is also disclosed. Compositions wherein cyclosporin is conjugated to an immunogenic carrier or a label, optionally through a linking group, at an alanine nitrogen atom of the cyclic backbone of cyclosporin are also disclosed. Compositions wherein atiocyclosporin is conjugated, optionally through a linking group, to an immunogenic carrier or a label are also disclosed. Where cyclosporin is conjugated to an immunogenic carrier, the conjugates may be used as immunogens for the preparation of antibodies which are capable of recognizing cyclosporin. Where atiocyclosporin is conjugated to an immunogenic carrier, the conjugates may be used as immunogens for the preparation of antibodies which are capable of recognizing interfering cross-reactive material but substantially incapable of recognizing cyclosporin or cyclosporin-label conjugates. Where cyclosporin is conjugated to a label, the conjugates may be used as part of a signal producing system in cyclosporin assays. Both the antibodies and label conjugates are useful in the disclosed assay methods.