The generation of an acidic pH in intracellular organelles is required for several membrane and protein recycling processes. For instance, the internalization of ligands by receptor-mediated endocytosis is followed by the development of an acidic pH inside endosomes; this allows dissociation of the ligand, which is then transported to the lysosomes, from the receptor, which is recycled to the cell surface. There is evidence that part of this recycling process involves the distal region of the Golgi complex, where terminal glycosylation occurs: when the plasma membrane transferrin receptor is desialylated by neuraminidase treatment, it acquires new sialic acid molecules after endocytosis and before cell-surface re-expression. Golgi membranes have been shown to contain a proton pump and the distal Golgi cisternae appear to have an acidic content. Here, we have studied the effects of chloroquine and ammonium chloride, which raise the pH of acidic intracellular compartments, on the processing and secretion of immunoglobulins by plasma cells. Sialic acid transfer to terminal galactose residues, a reaction known to occur in the distal Golgi shortly before secretion, is completely and rapidly inhibited in the presence of these drugs, without significant modification of the secretion rate. This effect is accompanied by a dilatation of the Golgi cisternae and is not rapidly reversible.