Cultured fibroblasts from mucolipidosis IV patients accumulated phospholipids when compared to normal controls or cells from other genotypes. The major stored compounds were identified as phosphatidylcholine, phosphatidylethanolamine and to a larger extent lysophosphatidylcholine and lysobisphosphatidic acid. Pulse chase experiments of 32P-labelled phospholipids showed increased retention of these compounds in the mucolipidosis IV lines throughout the pulse and chase periods. Phospholipase A1, A2, C, D and lysophospholipase showed normal activity in the mucolipidosis IV lines and thus the metabolic cause for this storage remains to be identified.