A variety of Agents (viz : cytochalasin D, colcemid, cytochalasin D+ colcemid, Ca2+-free medium, 7-ketocholesterol, cholesterol, concanavalin A, anti-embryonal carcinoma antiserum and tunicamycin) which modify the cell membrane and/or cytoskeleton were used to investigate the molecular and cellular basis of the intercellular and intracellular components of compaction and analyse the relationships between them. It was found that the individual components could be selectively dissociated from one another. Cell flattening and close intercellular apposition were the most sensitive features and affected by the majority of agents. Tight junctions did not form in the absence of intercellular apposition, however an apparently normal degree of intercellular apposition did not necessarily lead to the assembly of these junctions. Polarization of individual blastomeres, as assessed by the reorganization of the cell surface, was the component most resistant to experimental intervention since it occurred in the presence of all agents used, though it was modified by some of them. The result are discussed in terms of the molecular and cellular events underlying polarization, intercellular apposition and tight junction formation as well as the significance of these events for normal blastocyst formation.