Primaquine in a flowing solvent is photolyzed with a 200 W Xe-Hg arc lamp and the intensely fluorescent photochemical product is measured in a fluorimeter. The effect of solvent composition and pH, irradiation time, and excitation conditions are evaluated. Primaquine in blood serum is measured by introducing a sample of blood serum into the flowing stream and performing the photochemical-fluorescence measurement. The fluorescence background of blood serum was very low allowing a detection limit of less than 0.1 μg/mL. Excellent recoveries were obtained.