Prostereoisomerism (PIM), representing a subgroup within the achiral class of molecules, is manifested ina chiral environment. Although this is regardless of the orientation of the prostereoisomeric (PIC) molecule, PIM ismanifested most emphatically in relation to a chiral surface (`two-dimensional chirality'). Then PIM is tantamount to`de facto chirality' as attachment of the PIC molecule to the surface leads to diastereomeric possibilities (cf. Ogston'shypothesis). Furthermore, the formation of host{guest complexes requires a steric complementarity between the compo-nent molecules. The fact that the weak dispersive forces involved therein require an optimum distance implies a `snugnessof t' criterion. This further implies that a chiral host prefers a chiral guest molecule, and a chiral surface prefers a PICmolecule as substrate. These indicate a general stereochemical criterion for host{guest complexation that is particularlyrelevant to the case of biological receptors. Indeed, a survey of several known achiral drug molecules indicates that theygenerally possess at least one PIC moiety, lending credence to the above arguments. Thus, it would appear that PIMrepresents a maximum level of molecular symmetry for biological activity to be manifested efficiently.