The infection kinetics were determined for race CR3 of Erysiphe graminis f. sp. hordei on six sets of isolines of barley.  Lines with genes Ml-a, Ml-a6, or Ml-a13 gave a final infection type 0 whereas lines with genes Ml-a7(Mu), Ml-a7(LG), or Ml-a10 gave final infection types of 1-2 or 2.  The percent elongating secondary hyphae (ESH) ranged from 8 to 21 at 28 hr after inoculation.  A significant difference in the percent ESH was found among three groups of genes: (Ml-a, Ml-a6, Ml-a13), Ml-a7(Mu), and [Ml-a10, Ml-a7(LG)].  A significant difference was also found in the rate of development of ESH between Ml-a7(LG) and the other five genes examined, suggesting a difference in specificity.  It is proposed that the gene in cultivar Long Glumes at the Ml-a locus conferring resistance to race CR3 of E. graminis f. sp. hordei be designated Ml-a15.