Sub-parts per billion (ppb) levels of aqueous hydrogen peroxide have been determined with a flow injection analysis system employing a single bead string reactor composed of horseradish peroxidase covalently bound to an aminated macroporous polymeric absorbent with glutaraldehyde and a passive cation exchange membrane reactor to alter pH. The chemistry relies on the peroxidase mediated oxidation of nonfluorescent p-hydroxyphenylacetate to its fluorescent dimer. The advantage of the system includes rapid throughout rates (40 samples/h), excellent detection limits (0.3 ppb H2O2) and large dynamic range of linear response (1 ppb - 1 ppm). However, the immobilized enzyme is not useful for the analysis of organic peroxides which act as inhibitors. (Copyright (c) Springer-Verlag, 1986.)