Effects of soybean protein isolate (SPI) and related nitrogen (N) sources on the serum and liver cholesterol concentrations were investigated in chicks. SPI was hydrolyzed by endo-and exo-type microbial proteases for 24h, and obtained hydrolysate (HYS) was further segmented to digested soluble fraction (DSF) and undigested fraction (UDF) by centrifugation. Seven-day-old chicks were fed experimental diets containing 0.5% cholesterol for 10 days. In the first experiment, various N sources including casein were added at the N level equivalent to 5% crude protein (CP) to the basal diet containing 20% CP. SPI and HYS were equally effective in lowering the serum and liver cholesterol concentrations as compared with casein. In contrast, DSF as well as amino acid mixture simulating SPI increased the serum and liver cholesterol concentrations. UDF had the strongest cholesterol-lowering action among the N sources tested in the first experiment. In the second experiment, 1.2% UDF added to the basal diet was as effective as 6.0% SPI in lowering the serum and liver cholesterol concentrations, but 0.6% UDF had little effect. Since the ratio of DSF and UDF was 9:1 on the weight basis, 6.0% SPI (5.1% as CP) contained 0.6% UDF (0.3% as CP). These results indicate that UDF is an important determinant in cholesterol-lowering effect of SPI, although UDF was not able to account entirely for the cholesterol-lowering effect of SPI.