The accumulation of phosphatidylcholine (PC) in cultured fibroblasts of mucolipidosis IV (MLIV) patients was studied by subcellular fractionation on percoll gradients. Labelled PC accumulated in secondary lysosomes of the MLIV cells in significantly higher rates when compared to normal controls incubated with the precursors [32P]phosphate or [14C]choline. This accumulation was noted after 4 days of pulse and became more profound after 7 days of chase resulting in a 30-fold increase of this substance in the lysosomal fraction of MLIV compared to normal controls. On the other hand, no significant increase of radioactive PC was demonstrated in the buoyant fraction of the affected cells, and similarly the rate of disappearance of labeled PC from this fraction was identical in MLIV and controls. The retention of PC in lysosomes of MLIV could also be demonstrated following incubation of cultured fibroblasts with the radioactive phospholipid itself. In these experiments increased labelled PC in MLIV was also noted in endosomes, which are involved in the uptake process of PC enroute to the lysosomes. The metabolic defect leading to this storage in MLIV has not yet been identified, but these data indicate impairment in the lysosomal catabolism of phospholipids in MLIV.