Skin fibroblast cultures from patients with I-cell disease (mucolipidosis II) are characterized by multiple lysosomal enzyme deficiencies The present studies deal with the consequences of these deficiencies with respect to the metabolism of plasma low-density lipoproteins. Degradation of the protein moiety was defective in I-cells compared with control cells, but the binding and internalization of low density lipoprotein were much less affected. Measurements of low-density lipoprotein degradation in homogenates demonstrated directly for the first time a deficiency of acid proteinase activity in I-cell fibroblasts. Comparison of results in 6-h incubations with those in 24-h incubations showed accumulation of intracellular low-density lipoprotein in I-cell fibroblasts and an accelerating rate of degradation, possibly attributable to intracellular accumulation of low-density lipoprotein substrate. The significance of these findings with respect to low-density lipoprotein metabolism in vivo is discussed.