The advantages and disadvantages of Laser Nephelometry (LN) in the determination of IgD and IgE are reported. Two laser nephelometer models (Behringwerke/Marburg), different batches of LN cuvettes, WHO reference standard sera, rabbit anti-human antisera and randomly selected allergic patients' sera were used for the standardization of the method. Cuvette blank values were significantly lower in the new model of laser nephelometer and the precision of these measurements was very high when two different cuvette charges were compared. In the determination of IgE by LN, it was possible to detect levels down to 125 IU/ml, the accuracy of the estimations varying between 4.8 and 8.2% and the repeatability between 3.2 and 24.4%, the highest variation coefficient being obtained in low level samples. The overall agreement between LN and RIST in 55 serum samples was 71%, and at concentrations below 200 IU/ml (normal) and above 400 IU/ml (increased) 80% and 85% respectively. In the determination of IgD by LN, the accuracy of the estimations was also very good (2.4 to 7.8%) and the variation coefficient varied between 2.8 and 13.3%. In the comparison of IgD estimations with LN and radial immunodiffusion in 27 samples a correlation coefficient of r = 0.82 was obtained. Although normal adult IgE values cannot be analysed, the clinically important increased IgE levels are correctly determined by LN. The method is more sensitive than the Mancini technique for IgE determination and in comparison with RIST, though low values are not obtained, LN is quicker, simpler and cheaper.