Our data demonstrate that the uptake of surface Ia into an intracellular compartment of B lymphoma or normal spleen cells is limited to about 20% after 2 to 3 h. The extent of internalization does not vary with several types of stimulation, including LPS, phorbol esters, anti-Ig-plus phorbol ester-stimulated EL-4 T cell supernatant, and Con A supernatant. Resting and activated B cells had similar rates of internalization. The rate and extent of uptake of surface Ia molecules into an intracellular compartment was monitored quantitatively through the use of a mAb radiolabeled with 125I. The internalization of Ia molecules was compared to that of transferrin receptor, a receptor that undergoes rapid internalization and recycling and accumulates in a intracellular pool that can be trapped by monensin. The internalization of Ia was not affected by monensin, although its synthetic pathway is disturbed by this drug. The potential use of internalized Ia for formation of T cell-triggering complexes of Ia and Ag fragments is not ruled out by these data, but it appears unlikely that internalization provides the major mechanism permitting Ia interaction with Ag.