1. T lymphocytes and large granular lymphocytes internalized chylomicrons, very low-density lipoprotein, low-density lipoprotein, high-density lipoprotein and acetyl modified low-density lipoprotein through different receptors as assessed by flow cytometry. The observed internalization ranged from 8% to 20%. 2. All lipoproteins induced proliferative responses in T lymphocytes and large granular lymphocytes at optimum concentrations (40 micrograms of protein/ml for all lipoproteins except high-density lipoprotein). Chylomicrons, very low-density lipoprotein and low-density lipoprotein increased T-lymphocyte proliferative response by fourfold while inducing respectively a seven-, nine- and sevenfold increment in large granular lymphocytes. Similarly, high-density lipoprotein and acetyl modified low-density lipoprotein respectively induced a nine- and sevenfold increment in T cells and a 17- and eightfold increment in large granular lymphocyte proliferative response. 3. Both cell types internalized more lipoprotein when they were stimulated with interleukin 2. Chylomicrons and low-density lipoprotein internalization was increased threefold and very low-density lipoprotein internalization twofold, while high-density lipoprotein internalization was unchanged in both cell types. Acetyl modified low-density lipoprotein internalization was fourfold higher in large granular lymphocytes only. 4. The proliferative response of interleukin-2 stimulated cells was different from that of unstimulated cells. Chylomicrons and very low-density lipoprotein induced a sixfold increment in T-cell proliferative response but only a fourfold increment in large granular lymphocytes. Low-density lipoprotein and acetyl modified low-density lipoprotein induced respectively a sevenfold and eightfold increment in T cells and a eightfold and threefold increment in large granular lymphocyte proliferative response. High-density lipoprotein did not affect T-lymphocyte proliferative response while inducing a twofold increase in large granular lymphocytes. 5. Lipoproteins are important in the proliferative response of unstimulated and interleukin-2-stimulated cells.