MEDAWAR's differential 'mesoderm inhibitor', originally obtained from malt, ungerminated grain and oranges, has been closely reproduced through synthesis by Medawar, Robinson and Robinson1, who are not entirely certain whether their malt-distillate factor is optically active δ-hexenolactone or an allied substance. Because of structural similarity with panto-lactone, which combines with β-alanine to form pantothenic acid, synthetic dl-δ-Δαβ hexenolactone and its natural equivalent (probably parasorbic acid) were suspected of entering somehow into pantothenic acid metabolism, possibly by competition with pantolactone.