Background: Immune cells participate in the formation of atheromatous plate, however little is known about the effects of native or oxidatively modified lipoproteins on these cells. Aim: To study the effects of lipoproteins on in vitro mononuclear cell proliferation. Material and methods: Peripheral blood mononuclear cells were obtained from 10 patients with type 2 diabetes mellitus (aged 52 ± 9 years old with a disease duration of 8.2 ± 5.7 years and a mean glycosilated hemoglobin of 9.3 ± 2.2%) and 10 non diabetic healthy controls (aged 50.3 ± 7.1 years old). These were stimulated with phytohemagglutinin (PHA) alone or in the presence of native LDLS, malondialdehyde modified LDLs or glycated LDLs. Proliferation was measured as 3H-thymidine incorporation and expressed as Stimulation Index (SI). Results: SI of patients and healthy subjects, after PHA stimulation were similar: (57.5 ± 29.8 and 61.1 ± 23.5) respectively LDLs did not induce proliferation in neither group. Native LDLs produced a 98% inhibition of PHA induced proliferation. Malondialdehyde modified and glycated LDLs caused a 50% inhibition. The suppressive effect was maintained when lipoproteins were incorporated to culture media 60 min prior or after PHA stimulation. Conclusions: Lipoproteins inhibit in vitro PHA induced peripheral blood mononuclear cell proliferation both in diabetic and in non diabetic subjects.