Biosynthetic conversion of Ia oligomers from three chains (alpha, beta, gamma) to two (alpha, beta) before surface expression was inhibited in B lymphoid cells by treatment with chloroquine, resulting in the accumulation of Ia complexes composed of mature alpha and beta chains, and gamma chains at various states of sialylation. Other stages of Ia biosynthesis and processing appeared unaffected, indicating that chloroquine selectively interfered with the gamma chain dissociating mechanism itself. Similar effects were also observed with ammonium chloride. Because of the nature of such lysosomotropic agents, these results suggest that an intracellular acidic compartment may be involved in processing Ia oligomers to accomplish dissociation from gamma chains. Since chloroquine is known to inhibit Ia-restricted antigen presentation in accessory cells, our results raise the possibility that the pathways of antigen processing and Ia biosynthesis may use some common intracellular compartments.