Two selected mutant lines, C26 and C82, with highly significant 1000-seed weight and generated through irradiation treatment of Amaranthus cruentus L. 'Ficha', were evaluated and compared to an original nontreated control and reference variety Aztec. The phenotypic traits and biochemical properties of the amaranth seeds were studied. Comparable values for crude protein content, albumins, and globulins fractions and overall coefficients of nutritional quality were found in the evaluated samples. However, C26 showed significantly lower concentrations of celiac-related prolamins and glutelins in comparison with the control sample. Both of the mutagenesis-derived lines showed consistently superior performance of 1000-seed weight across two tested environments during a multiyear evaluation. C82 showed seed weight advantage over control seeds of Ficha and reference variety Aztec and it was released as a new amaranth variety named Pribina in 2013.