Abstract: A three-stream (0.6 mL min-1 each) system was used, containing three coils in series, of length 25, 50 and 100 cm (A, B and C, respectively). A sample (85 µL) was injected into a stream of reagent solution of pH 6.2 composed of 0.1 M hydroxylammonium chloride and 0.15% Brij-35 in aqueous 25% ethanol, the mixture was passed through reaction coil A (for hydrolysis of β-lactam rings), then acidified to pH ~1 by mixing with a stream of 0.5 M H2SO4 in coil B, merged with solution comprising 12.8 g of (NH4)2Fe(SO4)2 and 1 mL of 15% Brij-35 in 100 mL of 0.05 M H2SO4 - ethanol (3:1), and passed through coil C. The absorbance of the colored complex formed with Fe(III) was measured at 505 nm. Calibration graphs were rectilinear for 0.25 to 3 mg mL-1 of ampicillin, benzylpenicillin, procaine penicillin or phenoxymethylpenicillin, and the detection limits were ~0.1 mg mL-1. The coefficient of variation (n = 6 to 10) were 1.5% for ~1 mg mL-1 of individual penicillins. The method is suitable for analysis of pharmaceuticals; throughput is 50 h-1.