A method for making antibody conjugates for use in antibody screening assays comprising the steps of: providing a plurality of antibody-containing samples that vary with respect to antibody quantity and antibody sequence provided that, in a majority of the plurality of the antibody-containing samples, substantially all of the antibody present in a single sample is of the same sequence; immobilizing the antibodies on a solid support to provide a plurality of samples comprising immobilized antibodies; fully reducing the reducible disulfide bonds of the immobilized antibodies to provide a plurality of samples comprising reduced immobilized antibodies, wherein the reduction is selective for reducible disulfide bonds; and either a) reacting the reduced immobilized antibodies with capping agent, drug or drug-linker, and optionally a detection agent, to provide a plurality of samples comprising immobilized antibody conjugates, wherein the capping agent, drug or drug-linker, and optional detection agent selectively react with reactive thiols, the capping agent, drug or drug-linker, and optional detection agent are provided in molar excess, and the ratio of capping agent, drug or drug linker, and optional detection agent is selected so as to achieve a desired level of drug loading; or b) reacting the reduced immobilized antibodies with capping agent, and a detection agent to provide a plurality of samples comprising immobilized antibody conjugates, wherein the capping and detection agent selectively react with reactive thiols, wherein the capping agent and detection agent are provided in molar excess, and the ratio of capping agent and detection agent is selected so as to achieve a desired level of capping agent and/or detection agent loading; and eluting the antibody conjugates to provide a plurality of antibody conjugate compositions. providing a plurality of antibody-containing samples that vary with respect to antibody quantity and antibody sequence provided that, in a majority of the plurality of the antibody-containing samples, substantially all of the antibody present in a single sample is of the same sequence; immobilizing the antibodies on a solid support to provide a plurality of samples comprising immobilized antibodies; fully reducing the reducible disulfide bonds of the immobilized antibodies to provide a plurality of samples comprising reduced immobilized antibodies, wherein the reduction is selective for reducible disulfide bonds; and either a) reacting the reduced immobilized antibodies with capping agent, drug or drug-linker, and optionally a detection agent, to provide a plurality of samples comprising immobilized antibody conjugates, wherein the capping agent, drug or drug-linker, and optional detection agent selectively react with reactive thiols, the capping agent, drug or drug-linker, and optional detection agent are provided in molar excess, and the ratio of capping agent, drug or drug linker, and optional detection agent is selected so as to achieve a desired level of drug loading; or b) reacting the reduced immobilized antibodies with capping agent, and a detection agent to provide a plurality of samples comprising immobilized antibody conjugates, wherein the capping and detection agent selectively react with reactive thiols, wherein the capping agent and detection agent are provided in molar excess, and the ratio of capping agent and detection agent is selected so as to achieve a desired level of capping agent and/or detection agent loading; and a) reacting the reduced immobilized antibodies with capping agent, drug or drug-linker, and optionally a detection agent, to provide a plurality of samples comprising immobilized antibody conjugates, wherein the capping agent, drug or drug-linker, and optional detection agent selectively react with reactive thiols, the capping agent, drug or drug-linker, and optional detection agent are provided in molar excess, and the ratio of capping agent, drug or drug linker, and optional detection agent is selected so as to achieve a desired level of drug loading; or b) reacting the reduced immobilized antibodies with capping agent, and a detection agent to provide a plurality of samples comprising immobilized antibody conjugates, wherein the capping and detection agent selectively react with reactive thiols, wherein the capping agent and detection agent are provided in molar excess, and the ratio of capping agent and detection agent is selected so as to achieve a desired level of capping agent and/or detection agent loading; and eluting the antibody conjugates to provide a plurality of antibody conjugate compositions.