A heterogeneous on line electrochemical enzyme immunoassay has been developed for the antiasthmatic drug theophylline using a flow injection analysis system containing a protein A immunoreactor to separate antibody bound and unbound theophylline-alkaline phosphatase. In this assay the antibody, label and sample are sequentially immobilised onto the protein A immunoreactor. The antibody-protein A and antibody-protein A label/sample reaction takes place at near neutral pH, and the complexes are eluted at acid pH. p-Aminophenyl phosphate (PAPP) and p-aminophenol (PAP) have been used as the detection system. Experimental variations such as flow rate and reactant concentration have been studied and serum theophylline has been determined by this method and good r.s.d values and % recoveries have been achieved.