Analyses on functionality and quantity of secreted antibodies in four independent cell lines. Antibody expression in cell lines #3, #8, #11 and #12 was induced at time point T0 with a culture density of 0.65. Subsequently, samples of the medium were taken over three days (T1, T2, T3) followed by a medium exchange (T0*) and two additional days of sampling (T1*, T2*). Functionality of the antibody, i.e. the efficiency of the antibody to bind to its antigen, was measured by ELISA assays. The secreted antibody binds to the antigen with an efficiency that basically correlates with the antibody amounts measured in the samples. During the first two days antibody quantities within the medium increased to concentrations of 450–850 ng/ml. After day two the amount of antibody in the medium slightly decreased, which is consequently accompanied by a loss in binding efficiency. The removal of the antibody containing medium and addition of fresh nitrate-containing medium restored antibody expression/secretion and resulted in antibody concentrations of 1550–2550 ng/ml at day T2* depending on the cell line tested. Error bars indicate standard deviation (n = 3).