Abstract: The electrode was a 0.25-mm diameter copper wire with only the tip exposed; it was mounted in a flow-cell that incorporated a double-junction silver - AgCl reference electrode and also a platinum-wire auxiliary electrode for cathodic pre-cleaning of the copper electrode. The protein was injected (injection volume 0.2 ml) into a carrier stream (1 mL min-1) of Cu(II) in 0.05 M phosphate buffer of pH 7.0; with 10 µM-Cu(II), proteins could be determined in the concentration. range 0.1 to 1 µM, the limits of detection for human albumin, bovine albumin, ovalbumin, phosvitin, human γ-globulin and catalase being 40, 90, 100, 30, 200 and 4 µg mL-1, respectively. Chloride only interferes when present at >0.01M, as compared with >10 µM in steady-state measurements. The system has been applied to proteins eluted from a size-exclusion column.