A study has been made of the advantages derived from blending small quantities of guayule rubbers with the allpurpose synthetic rubber, GR-S. The low er resin guayule rubbers, used in amounts up to 20% of the total rubber, THE study presented here was performed during the latter part of the war, when natural rubber continued to be the most critical of all strategic materials (IS) and the question as to how our limited supply of natural rubber could be used to best advantage was of great importance.With the end of the war the importance of this particular question greatly diminished, but the mixing of natural and synthetic rubbers to obtain superior blends is still of present and future interest. The future situation in regard to rubber cannot be accurately predicted, but both synthetic and natural rubbers will be used. Judging from developments in this country, Mexico, Russia, and Argentina, it is likely that guayule will remain in production for its special properties of plasticity and tack.GR-S is particularly inferior to Hevea in processing characteristics, tack, tear resistance, cut-growth resistance, tensile strength at high temperatures, and heat build-up. One method for alleviating this inferiority is to blend small quantities of natural rubber with GR-S to improve the processing of the GR-S and improve some of the physical properties of its vulcanízales. This was first realized by the Bureau of Ships of the Navy Department; early in 1942 this bureau urged full cooperation between rubber manufacturers and interested government agencies to provide information on the compatibility of various types of guayule rubbers with Hevea rubber, -with various types of reclaimed rubber, and with all of the available synthetic rubbers. When Mexican or domestic resinous and deresinated guayule rubbers were substituted for GR-S to the extent of 20%, a striking improvement in processability, tack, tear resistance, and cut growth was observed by Morris and co-workers (10,11, IS). Carlton and Reinbold (7) showed that a blend of 25% Hevea and 75% GR-S had considerably better building tack, tensile strength, elongation, and tear resistance, and less heat build-up, than GR-S alone. Cohan and Myerson (9) found that as small an amount as 5% of domestic resinous guayule rubber gave improved processability, tack, tensile strength, and resistance to flex cracking.The work reported here was initiated to investigate further the subject of GR-S-guayule blends in a typical tread stock, and to study some types of guayule rubber which had not been used for this purpose before. All tests -were performed on a comparative basis; the standards for comparison were similar stocks compounded from GR-S blended with a good grade of Hevea smoked sheet. TYPES OF RUBBERThe types of guayule rubber studied and the kind of rubber used as a standard are shown in Table I. The samples are similar to but not identical with those used in a previous compounding study (8). For convenience a symbol and an abbreviated name are assigned to each rubber. The abbrevi...