Fluid energy milling and wet sieving were used to produce narrow particle size distributions (PSD) of fine lactose in the size range size <40 mu m. Fluid energy milling (K-tron Soder, USA) occurred at milling pressures of 552 and 690 kPa and feed rates of 400 to 1600 rpm. Wet sieving used 1-butanol pre-saturated with lactose to classify lactose into fractions of nominally < 5 mu m, 5 -10 mu m, 10-20 mu m and 20-45 mu m. The sonicator was positioned 3 mm above the surface of the sieve during wet sieving. PSD of the lactose fractions were measured by laser diffraction (Malvern MasterSizer S, Malvern Instrument Ltd., U.K.). Fluid energy milling produced a range of mono-modal distributions of lactose with VMD from 2.9 mu m to 41.7 mu m. The distributions of milled lactose (VMD of 5.5, 20.2 and 3 1.1 mu m) were broad with spans ranging from 1.6 to 12.4. Wet sieving required the use of sonication to achieve classification. Wet sieving removed fine particles <5 mu m in the milled lactose fractions of 5.5 mu m and 20.2 mu m and 31.1 mu m by 60.2%, 90.6% and 95% respectively after 15 repeat wet sieved cycles giving narrow distributions with spans in the range of 0.9-1.6. DSC and PXRD results for both milled and wet sieved lactose were consistent with those of alpha-lactose monohydrate and beta-lactose was not detected. (C) 2007 Elsevier B.V. All rights reserved.