The paper radioimmunosorbent test (PRIST) has been described as the method of choice for determining low serum IgE levels; the radioimmunosorbent test (RIST) as the method of choice for determining normal and elevated serum IgE levels. By replacing the 125I-labelled anti-IgE antibody used in the paper radioimmunosorbent test by the 125I-labelled anti-IgE reagent used in radioallergosorbent test (RAST) and by changing the serum dilution and the incubation time, this modified sandwich technique (MST) became comparable to the RIST in the normal and elevated IgE-region and showed results similar to the PRIST in the very low IgE-region. The affinity of the 125I-labelled anti-IgE of the RAST proved to be about 2.6 times higher than the antibody used in the PRIST, which explains the improved results in the normal and the good results in the very low IgE-region. The lowest serum IgE level measurable by this method was as low as 0.05 I.U./ml, as determined in 20 cord sera. The mean IgE level in cord sera was 0.45 I.U./ml (range 0.05--2.63 I.U./ml). The results of this study suggest replacement of the antiserum used in PRIST by the one available for RAST.