We have evaluated the analytic performance of a new immunonephelometric method developed for the measurement of amniotic fluid alpha-fetoprotein (AF-AFP) and compared it with the classic immunoradiometric method used up to now. Intrassay precision, evaluated for three AF-AFP levels, was good and Interassay imprecision shows higher coefficients of variation (CVs) especially for medium and high AF-AFP levels. Linearity was demonstrated for level higher than 300,000 ng/ml and no hook effects was observed. Comparison with an IRMA method gave the following regression equation: y = 3050 + 0.52X (r = 0.76, p less than 0.001, n = 36).