MLPP is a FORTRAN program set up to calculate the molecular lipophilicity potential in proteins and which has been imported into Insight II and Chem-X, the molecular modelling and graphics programs of Biosym Technologies and Chemical Design Limited. MLPP uses two bases of hydrophobicity atomic contributions—the base of 222 values and an extended and more precise base of 1350 values, and several optional distance functions. The program can be considered as an alternative to HINT (hydrophobic interactions) implemented in comparative molecular field analysis (CoMFA). Illustrations have been chosen for a globular protein, renin, and for two membrane proteins, bacteriorhodopsin and porin.