Exosomes are nanosized vesicles that are abundant in biological fluids. In recent years, exosomes have attracted increasing attention as their cargo may provide promising biomarkers for the early diagnosis of and therapy for many diseases, such as cancer. In addition to ultracentrifugation (UC), many alternative methods including size-exclusion chromatography (SEC) have been developed for isolating exosomes. It has been reported that the SEC method provided improved performance relative to the UC method in isolating exosomes from plasma, where the former contained less residual blood protein contamination. We have compared the SEC method with an optimized UC method in isolating exosomes from human serum. This was based on dilution of the serum to reduce the viscosity and a prolonged cycle of UC, followed by another four cycles. We found that >95% of serum proteins were removed without a significant loss of exosome proteins relative to SEC. We also combined one cycle of UC with SEC and found that this method provided improved results relative to the SEC method, although the serum protein contamination was several times higher than that of our optimized UC method. The TEM showed that the size distribution of exosomes isolated from each of the three methods was similar.