AbstractDifferent forms of lactose are available for direct compression of tablets. The use of spray-dried lactose, which has good flow and compressibility characteristics, is limited by its stability when stored under humid conditions. Sieved crystalline fractions ofα-lactose monohydrate such as the 100 mesh fraction, have very good flow properties and an outstanding stability, but the compressibility is so poor, that it can be used only in combination with other filler-binders, like microcrystalline cellulose. A third form of lactose, increasingly used in direct compression is anhydrous lactose. The commercially available products generally consist of an excess ofβnext toα-lactose. They both have good binding and stability characteristics, but a flowability which is less than optimum. The latter is caused by the rather irregular particle shape and the relatively high amount of fines. A newly developed form of lactose is anhydrousα-lactose. It is prepared by dehydration ofα-lactose monohydrate. Binding, flow and stability properties of this excipient were compared with the properties of other filler/binders. The results show that the compressibility of anhydrousα-lactose was about the same as that of anhydrousβ-lactose.The flow properties of anhydrousα-lactose were even better than the very good fluidity ofα-lactose monohydrate 100 mesh. At storage under normal or humid conditions, there was no change in hardness of tablets compressed from anhydrousα-lactose.A comparative evaluation of the effect of mixing with magnesium stearate on the binding properties of filler/binders showed that all the lactose products investigated, including anhydrousα-lactose, behave in an intermediate manner, between complete plastic deformation and complete brittle fracture. For this reason there is a limited decrease in crushing strength for tablets compressed from anhydrousα-lactose, during mixing with magnesium stearate.Some formulation examples will show that anhydrousα-lactose is a very useful filler/binder in direct compression, of which tablets with a low weight variation, sufficient strength, a low friability, a fast disintegration and a high drug release can be prepared.