AbstractMethisoprinol (isoprinosine) is a synthetic compound with reported antiviral and immunomodulating properties. Results of the present study showed that methisoprinol at concentrations ≥ 200 μg/ml reduces the p24 and gp120 viral antigen expression on the surface of human immunodeficiency virus (HIV)‐infected lymphocytes and the reverse transcriptase levels. In addition, cell viability, the number of the CD4 + cells, and the CD4 + /CD8+ cell ratio are higher in methisoprinol‐pretreated cell suspensions than in untreated HIV‐infected cell cultures. A quantitative freeze‐fracture study on the density of the intramembranous particles (IMP) present on both fracture faces of the plasma membrane of lymphocytes has shown that pretreatment with methisoprinol induces a different molecular organization resulting in a nearly three times increase of I