Lunasin is a 43-amino acid bioactive peptide from soybean and other plant sources which is reported to possess anti-inflammatory and anticancer properties. The objective of this study was to assess the presence and concentration of lunasin in blood of men fed soy protein products. Five healthy male subjects who were 18-25 years old consumed 50 g of soy protein for 5 days, and blood was taken 30 min and 1 h after soy protein ingestion on day 5. Lunasin was isolated from plasma using strong anion exchange beads in a magnetic particle concentrator and eluted with 20 mM triethanolamine at pH 8.0 with 0.20 M NaCl. The concentration of lunasin in plasma as determined by an enzyme-linked immunosorbent assay ranged in the various subjects from 50.2 to 110.6 ng/mL of plasma (average +/- standard deviation, 66.0 +/- 25.4 ng/mL) for blood taken at 30 min and from 33.5 to 122.7 ng/mL of plasma (71.0 +/- 32.8 ng/mL) for blood withdrawn 1 h after ingestion on day 5. We estimated an average of 4.5% absorption (range of 2.2-7.8%) of lunasin from the total lunasin ingested from 50 g of soy protein. Matrix-assisted laser desorption ionization time-of-flight peptide mass mapping showed that a 5 kDa peptide similar to synthetic lunasin was present in plasma samples of people who consumed soy protein while absent at the baseline plasma samples from the same individuals. Liquid chromatography-tandem mass spectrometry analysis showed the presence of amino acid sequences from lunasin in plasma samples after soy intake for 30 min and 1 h. No peptides from lunasin were present in plasma samples without soy intake. The results of this study suggest that lunasin is bioavailable in humans, an important requirement for its anticancer potential.