The growth properties and cytopathic effects of several HIV-1 and HIV-2 strains were compared between cultures on human lymphocytes and monocytes/macrophages, respectively. For some isolates (among these three paired isolates from blood and cerebrospinal fluid) replication and cytopathogenicity were comparable between lymphocytes and monocytes/macrophages (dual tropic viruses), while others showed a very specific tropism for only one cell type. Yet another subtype grew neither well on lymphocytes nor on macrophages. Taking into account the growth properties in monocytes/macrophages we propose a classification system for HIV subtypes on these cells (alpha-delta), in analogy to the nomenclature for HIV-subtyping on lymphocytes (a-d). Using this system, some prototypic viruses (LAV/HTLV-IIIB, HIV-2ROD, SIVBK28, HIV-2ALT) as well as several other HIV-1 and HIV-2 isolates were subtyped.