Lactose is available in several crystalline forms, which differ in binding properties. A new method of estimating the fragmentation propensity was applied to investigate the consolidation and compaction behaviour of this excipient for direct compression. Mercury porosimetry was used to demonstrate that crystalline lactose fragments during compaction. Tablet strength was found to be dependent on the degree of fragmentation only. This finding indicates that the nature of the actual binding must be the same for the different types of crystalline lactose.