The number and distribution of intramembrane particles (IMPs) were examined on capped lymphocytes and control cells. Simultaneous movement of surface receptors and microvilli during ligand-independent (LI-) cap formation in a hypertonic medium made it possible not only to dis-tinguish between capped and non-capped cells in freeze-fractured specimens but also to detect the region where receptors had accumulated by referring only to their morphology. No accumulation of IMPs in the region of the cap was detected, and the distribution of IMPs was random for both capped and non-capped cells. However, the density of IMPs on the protoplasmic halves of cleaved membranes was slightly, but significantly, lower for capped cells than for non-capped cells, whereas that on the exoplasmic halves did not differ. These results suggest that the majority of IMPs are not composed of mobile receptors such as surface Ig, and surface receptors found as IMPs before cap formation might be changed by this event in the intercalated state into the lipid bilayer and, thus might not be detected as IMPs on cap forming cells.