The aim of the study was to evaluate the nutritional value of Lathyrus sativus var. Krab in broiler chickens as well as that of 3 mutants (K-12, K-63 and K-64) developed from var. Krab by mutagenic treatment and selection towards improved agronomic traits. The proximal composition of mutants was similar to that of var. Krab with the exception of a higher protein content in K-12, and lower β-N-oxalyl-L-α,β-diaminopropionic acid (ODAP) and tannin contents in the mutants. Trypsin inhibitor activity ranged from 57 to 68 mg/g crude protein (CP) in mutants, and equaled 72 mg/g CP in Krab variety seeds. Four test diets composed using the basal diet and evaluated grasspea seeds at a proportion of 6:4 were used in the balance experiment; the control diet without grasspea, and 4 isonitrogenous diets containing 10% grasspea were used in the growth experiment. Apparent total tract protein digestibility and metabolizable energy in mutant K-12 equaled 72% and 9.8 MJ ME/kg DM, respectively, and were similar as in var. Krab but higher than in mutants K-63 and K-64 (P<0.05). In chickens fed from day 8 to 40 of age diets containing 10% grasspea, performance, tibia ash content, and tibia ultimate strength did not differ from the control group. However, birds fed the diet with var. Krab seeds had enlarged pancreases (P<0.05) and livers in comparison with controls; the weight of these organs was lower (P<0.05) in groups fed mutant seeds. Bone and joint deformations indicative of neurolathyrism were found in four birds from the group fed var. Krab, while they were not seen in birds fed the diets with seeds from mutants. The results indicate that mutagenic treatment of Lathyrus sativus can have a positive effect on the nutritional quality of seeds.