Rapid immunochemical assays were developed for the determination of apolipoproteins A-1 and B in serum. The methods are based on the measurement of immunoprecipitation at a wavelength of 340 nm. Both assays are easy to perform, their reproducibilities are good (the coefficients of variation for within-run and between-run imprecisions are below 5%), and their correlations with radial immunodiffusion method are high (r = 0.96). The effect of storage on native serum specimens was also studied. Apolipoprotein B is stable at least 4 weeks in a refrigerator but in some specimens the level of apolipoprotein A-1 rises nearly 10% during this time. Repeated freezing and thawing, on the other hand, may have a harmful effect, especially on apolipoprotein B. Therefore, use of fresh serum specimens is recommended for both assays. The present method is well suited for routine measurements because it can be easily adapted to most clinical chemistry analysers.