The optimum fluorescence conditions for erbium (III) are obtained by irradiating this lanthanide at 435 nm in 0.04 mug ml(-1) diantipyrylmethane solution at pH = 8 (lambda (em) = 510 nm). The method proposed is satisfactory for the determination of erbium (III) in the range of 0.001 to 1 mug ml(-1). The relative standard deviation 0.02 mug ml(-1) Er (III) in 0.04 mug ml(-1) diantipyrylmethane solution is 1.1%. The effect of other rare earths upon the intensity of the fluorescence emitted by erbium (III) is discussed. (C) 2001 Elsevier Science B.V. All rights reserved.