Available lysine values in protein feedstuffs were determined by rat bioassay (RBA) using the slope-ratio method. Slope-ratios were calculated by regression analysis of carcass weight gain according to percent of lysine supplied by the test protein feedstuff(RBA-A), live weight gain according to percent of lysine supplied by the test protein feedstuff(RBA-B), carctrss weight gain according to the amount of lysine intake from the test protein feedstuff(RBA-C), live weight gain according to the amount of lysine intake from the test protein feedstuff(RBAD), FCR(carcass weight gainfeedj according to percent of lysine supplied by the test protein feedstuff(RBA-E) and FCR(live weight gain/feed) according to percent of lysine supplied by the test protein feedstuff(RBA-F). Average lysine availability as detemlined by various RBA methods was highest in fish meal(133.0%), followed by can gluten(108.0%), soybean meal(97.9%), canola meal(83.9%), meat and bone meal(68.0%), cottonseed meal (58.7%), rapeseed meal(30.7%) and leather meal(15.9%). Lysine availabilities based on lysine intake(RBA-C, D) and live weight gain(RBA-B, D) tended to be higher than those haled on percent of lysine(RBA-A, B) and carcass weight gain(RBA-A. C), respectively. Lysine availability based on live weight gain had a higher correlation with chemically determined lysine availability than that based on carcass weight gain. FDNB(1-fluoro-2, 4-dinitrobenzene) available lysine showed a higher correlation with available lysine as determined by RBA than TNBS(2, 4, 6-trinitrobenzene sulfonic acid) availahle lysine did.