Lipoproteins chylomicrons (CM) very low density lipoprotein (VLDL). Low density lipoprotein (LDL) high density lipoprotein (HDL) and acetyl modified LDL (AcLDL) are able to stimulate macrophages upon binding lo specific receptors. Using flow cytometry and a specific monoclonal antibody against phosphotyrosine. We assessed the effect of lipoproteins on total intracellular expression of immunoreactive phosphotyrosine. Tyrosine phosphorylation increased when lipoproteins were added to a macrophage cell line which was previously cultured in RPM-0.50% fatty acid free bovine serum albumin. The optimal effect occurred at 15 min for all the lipoproteins used. The concentrations of lipoprotein were 10 μg of protein/ml of CM and 20 μg of protein/ml for the other lipoproteins. The effect of lipoproteins on phosphotyrosine positiveness was as follows: AcLDL> HDL > CM > VLDL > DL. Thus lipoproteins may activate macrophages through tyrosine phosphorylation and this signal transduction pathway seems to be enhanced when these cells are activated with AclDL.