The distribution of cation-independent (CI) mannose 6-phosphate (Man6P) receptors for lysosomal enzymes within the Golgi complexes of human fibroblasts has been investigated. In normal skin fibroblasts, CI Man6P receptors were localized by immunocytochemistry to cisternal elements of the Golgi complex which were found on only one side of the stack. A similar distribution of receptors was seen in fibroblasts from patients with mucolipidosis II (I-cell disease fibroblasts), cells which cannot construct Man6P residues on newly synthesized lysosomal enzymes and thus lack endogenous ligands for Man6P receptors. This receptor-enriched cisternae appeared set apart from the Golgi stack proper, could often be seen to extend from one apparently separate Golgi stack to another, had protrusions and vesicles with clathrin-like coats, and morphologically resembled the trans Golgi network (reticulum). Treatment of both cell types with the carboxylic ionophore, monensin, before fixation and immunostaining, resulted in the generation of dilated vacuoles on one side of the Golgi stack which contained immunoreactive Man6P receptors. The remaining, flattened Golgi cisternae were uneffected by the monensin treatment and did not exhibit any immunoreaction product. Fibroblast membranes were fractionated by sucrose-gradient centrifugation to partially separate Golgi membranes into cis, medial, and trans elements, and the results indicated that membranes enriched in Man6P receptors from both normal and I-cells migrated with markers of trans Golgi membranes and not with markers of cis or middle elements. It is concluded that Man6P receptors reside concentrated in trans Golgi cisternae, probably within elements of the trans Golgi network, in both normal and I-cell disease fibroblasts. Also, because no difference was seen between normal and I-cell disease fibroblasts, the trans Golgi must serve as a reservoir for Man6P receptors whether or not the receptors are involved in the transport of newly synthesized lysosomal enzymes in human fibroblasts.