Restoration in the msm1 cytoplasm of barley (Hordeum vulgare L. s.l.) was studied from the standpoint of population biology and physiological effects on kernel protein. Restorer genes of 82 accessions of wild barley (ssp. spontaneum) from Israel were determined. 38% of the accessions were maintainers of sterility, 48% were partial restorers, and 14% were restorers. Fourteen dominant restorer genes are described, and evidence for three cases of allelism to Rfm1a is presented. The restorer accessions and their designated gene symbols are: PI 282636 (Rfm,,e), PI 282637 (Rfm,f), PI 282646 (Rfm,,g), PI 284742 (Rfm,h), PI 284743 (Rfm,,i), PI 284753 (Rfm,,j), PI 284755 (Rfm1d), PI 296838 (Rfm,,k), PI 296850 isolate 16/7 (Rfm,,l), PI 296853 (Rfm,,m), PI 296856 (Rfm1b), PI 296899 (Rfm,,n), PI 296919 (Rfm1c), PI 296944 (Rfm,,o). PI 296850 was found to contain both a restorer and a non-restorer genotype. None of the PI accessions with a restorer gene is a carrier of an msm1-type male sterilizing cytoplasm. In the present sample, plants with restoration ability occurred with a higher frequency in the material from the Judean Foothills than that from the other regions of Israel. The greater adaptive value of plants with restoration ability on certain soil associations in semiarid and subhumic climate is suggested. The considerable frequency of restorers and partial restorers in male fertile cytoplasm suggests that the restoration system evolved before the msm1-type cytoplasm. In the nuclear genotype near-isogenic with either 'Adorra' or 'Risø 1508', msm1 plants heterozygous for Rfm1a produced 98.6 or 98.5% of the protein content in the respective recurrent pollen parent varieties. The amino acid compositions of the derivatives differed little from those of the varieties. In the derivatives, a consistent decrease was found in tryptophan, and consistent increases in isoleucine, phenyalanine, lysine, histidine, and arginine. In relation to glucose consumption, the bioenergetic cost calculated for the amino acid patterns found in the restored msm1 derivatives was slightly higher than that for the near-isogenic pollen parent varieties. The results suggest that the restorer gene in the heterozygous state normalizes the physiology of msm1 cytoplasm to a great extent.