We examined the hypothesis that the accumulation of phospholipid in cells exposed to gentamicin is due to impaired degradation. Experiments were performed in rabbit proximal tubular cells grown in primary culture. Cells exposed to 10(-3) M gentamicin manifested myeloid body formation and a progressive increase in total phospholipid that by day 6 was 44% higher than that of control cells and reflected increases of phosphatidylinositol of 235%, phosphatidylcholine of 60%, phosphatidylethanolamine of 90%, and phosphatidylserine of 55% above control values. Gentamicin impaired the degradation of these phospholipids. The t1/2 of the phospholipid pool labeled with [3H]myoinositol increased 146% from 1.17 (control) to 2.88 days (gentamicin); the t1/2 of the [3H]choline pool increased 34% from 1.77 to 2.38 days; the t1/2 of the [3H]ethanolamine pool increased 57% from 3.14 to 4.93 days; the t1/2 of the [3H] serine pool increased 37% from 6.30 to 8.63 days. Exposure of cells to gentamicin for 2 days also stimulated increased incorporation of [3H]myoinositol (68%) and [3H]ethanolamine (59%) into phospholipid. The data are consistent with the hypothesis that gentamicin inhibits the activity of lysosomal phospholipases that results in the accumulation of phospholipid within the lysosome in the form of myeloid bodies. Increased phospholipid synthesis may represent a compensatory response to the impaired lysosomal degradation of phospholipid. We postulate that the preferential increase of phosphatidylinositol reflects the capacity of the polycationic gentamicin to interact electrostatically with the anionic phosphoinositides and inhibit their turnover.