A method of determining a presence and/or amount of a small molecule having a molecular weight of about 500 to about 2,500, said small molecule being an immunosuppressant drug comprising a 15 - 50 membered ring, in a sample suspected of containing the small molecule, wherein the immunosuppressant drug is sirolimus the method comprising: (a) providing in combination in a medium: (i) the sample, (ii) a first binding partner that binds to and is specific for a first portion of the small molecule, wherein the first binding partner is a monoclonal antibody which exhibits substantially no binding affinity for tacrolimus, and (iii) a second binding partner that binds to the small molecule at a second portion of the small molecule other than the first portion, wherein the second binding partner is prepared from an immunogen that comprises a hapten that is not the small molecule or a derivative of the small molecule wherein the hapten comprises a moiety that is structurally similar to that of the second portion of the small molecule, wherein both the first binding partner and the second binding partner are monoclonal antibodies, and wherein the second monoclonal antibody is an anti-tacrolimus antibody and is prepared and selected for the binding of both tacrolimus and sirolimus. (b) incubating the medium under conditions for binding of the first binding partner and the second binding partner to the small molecule, and (c) examining the medium for the presence of an immunocomplex comprising the small molecule, the first binding partner and the second binding partner, the presence and/or amount of the immunocomplex indicating the presence and/or amount of the small molecule in the sample. (a) providing in combination in a medium: (i) the sample, (ii) a first binding partner that binds to and is specific for a first portion of the small molecule, wherein the first binding partner is a monoclonal antibody which exhibits substantially no binding affinity for tacrolimus, and (iii) a second binding partner that binds to the small molecule at a second portion of the small molecule other than the first portion, wherein the second binding partner is prepared from an immunogen that comprises a hapten that is not the small molecule or a derivative of the small molecule wherein the hapten comprises a moiety that is structurally similar to that of the second portion of the small molecule, wherein both the first binding partner and the second binding partner are monoclonal antibodies, and wherein the second monoclonal antibody is an anti-tacrolimus antibody and is prepared and selected for the binding of both tacrolimus and sirolimus. (i) the sample, (ii) a first binding partner that binds to and is specific for a first portion of the small molecule, wherein the first binding partner is a monoclonal antibody which exhibits substantially no binding affinity for tacrolimus, and (iii) a second binding partner that binds to the small molecule at a second portion of the small molecule other than the first portion, wherein the second binding partner is prepared from an immunogen that comprises a hapten that is not the small molecule or a derivative of the small molecule wherein the hapten comprises a moiety that is structurally similar to that of the second portion of the small molecule, wherein both the first binding partner and the second binding partner are monoclonal antibodies, and wherein the second monoclonal antibody is an anti-tacrolimus antibody and is prepared and selected for the binding of both tacrolimus and sirolimus. (b) incubating the medium under conditions for binding of the first binding partner and the second binding partner to the small molecule, and (c) examining the medium for the presence of an immunocomplex comprising the small molecule, the first binding partner and the second binding partner, the presence and/or amount of the immunocomplex indicating the presence and/or amount of the small molecule in the sample.