The confinement of macromolecules within enclosures or "pores" of comparable dimensions results in significant size- and shape-dependent alterations of macromolecular chemical potential and reactivity. Calculations of the magnitude of this effect for model particles of different shapes in model enclosures of different shapes were carried out using hard particle partition theory developed by Giddings et al. (J. Phys. Chem. 1968. 72:4397-4408). Results obtained indicate that the equilibrium constants of reactions, such as isomerization, self-association, and site binding, that result in significant change in macromolecular size, shape, and/or mobility may be altered within pores by as much as several orders of magnitude relative to the value in the unbounded or bulk phase. Confinement also produces a substantial size-dependent outward force on the walls of an enclosure. These results are likely to be important within the fluid phase of biological media, such as the cytoplasm of eukaryotic cells, containing significant volume fractions of large fibrous structures (e.g., the cytomatrix).