Efficient basic oxygen steelmaking (BOS) depends on the rapid formation of a highly basic slag and, hence, on the properties of the lime (CaO) added as a flux during BOS converter operation. To evaluate the factors influencing the quality of lime supplied to the steelplants at Port Talbot and Llanwern, laboratory studies and plant trials were undertaken on limestone samples from two different quarries at Shapfell and Batts Combe. Despite significant differences in the microstructures of the two limestones, the rates of weight loss with time were similar at the same calcination temperatures, as was the average crystal size for the lime-crystal aggregates produced under the identical calcination conditions. Moreover, the predicted rates of slag formation appeared comparable, as judged by the results derived from water reactivity tests completed for lime obtained under a wide range of calcination conditions for both limestone samples. In contrast, for equivalent calcination treatments, the porosity values and the amounts of fines generated during abraidability tests were lower for lime samples derived from Shapfell than from Batts Combe limestone. Although the commercial limes supplied from both sources meet the composition specifications set by British Steel, it is proposed that the higher impurity levels in the Shapfell limestone create larger quantities of intergranular viscous silicates during calcination, improving the sintering characteristics of the calcined lime. This view is supported by the observation that, during calcination of Batts Combe limestone, surface 'doping' of the lime with sodium carbonate resulted in a discernible reduction in the percentage of fines generated during subsequent abraidability experiments. In line with the laboratory-based results, plant trials indicated that the slag/metal distributions of sulphur and phosphorous, as well as slag basicities were comparable for commercial lime from the two suppliers. However, in agreement with the abraidability data, dust losses from the converters were substantially greater using the Batts Combe lime.