As the construction industry adapts to a global martketplace and growing environmental regulations, the efficient usage of materials and energy becomes evermore important. Many years ago  and much more recently , it has been suggested that in low water-to-cement (w/c) ratio concretes, some cement could be saved by replacing the "coarser" cement particles with relatively inert fillers, such as ground sand  or limestone powder . The addition of limestone powder to cement is a common practice in Europe, but is not yet commonplace in the U.S. The most recent version of the ASTM C150-04 standard specification for Portland cement  now permits limestone additions up to 5 %. Computer simulations have suggested that this replacement can be made in low w/c ratio concretes with little if any detrimental effects on performance, in terms of hydration and compressive strength development . This communication presents the results of a preliminary experimental study conducted to directly verify this hypothesis. Further support for this concept can be found in the recent work of Bonavetti et al.  who studied an ordinary portland cement and two interground limestone cements in pastes and concretes at a variety of water-to-cementitious materials ratios, and showed little loss in 28 d strength for the blended systems. In the study to be presented here, the coarser cement particles are manually replaced by similarly sized limestone as opposed to intergrinding the two materials.