Enzyme immunoassay for the quantitative determination of free therapeutic anti-TNF alpha antibodies (e.g. Infliximab/Remicade) in EDTA-plasma and serum. This method plots raw measurements of the standards versus concentration using linear (y) and log (x) axes and performs a 4-parameter logistic fit. Concentrations of samples (µg/ml) are determined from the fit and any specified dilution factors are applied.