The therapeutic use of antibodies has grown exponentially, providing new treatment options for various diseases. Monitoring treatment with therapeutic antibodies is a particular challenge because often the target antigen is no longer measurable or the results are unreliable. To overcome this problem, a recovery-ELISA was developed to quantify therapeutic antibody and antigen by a modification of the traditional sandwich immunoassay using omalizumab as an example. Standard serum samples were spiked with IgE and omalizumab in a certain concentration range to create standard curves. After incubation and washout procedures, the reaction was stopped and the plate read with bichromatic absorbance. This approach defines reference functions for serum IgE without and with antibody, respectively. Based on these functions, free IgE and omalizumab concentrations can be calculated in serum samples of patients treated with omalizumab. The recovery-ELISA allows us to easily quantify antigen (free IgE) and therapeutic antibody (omalizumab) simultaneously in the same sample and thus to monitor patients treated with the specific antibody.