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 [1] and much more recently [2], 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 [1] or limestone powder [2]. 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 [3] 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 [2]. 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. [4] 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.