Summary A programme of tests was undertaken of concrete prisms in axial compression to study the influence of the static strength and the maximum size of coarse aggregate upon the fatigue strength at one million cycles. Four types of concrete were tested having average static strengths of about 6,000 and 8,500 lb/in2, each made with aggregate of ¾ and ⅜ in. maximum size. The fatigue strength varied between 66 and 71 % of the static strength and, although the percentage values were significantly lower with the higher-strength concrete and with the ⅜ in. aggregate, the actual magnitude of the differences was small. The general strain history was typical of previous fatigue tests, and the ‘run-out’ specimens showed a marked increase in static strength over similar specimens that had not been loaded.