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Liposomes bearing surface-attached antibody (L-Ab) molecules can be used for various purposes including the immunospecific delivery of drugs or other materials to antigenic target cells. In this study, L-Ab were prepared to deliver an anti-human immunodeficiency virus (HIV) drug, dideoxycytidine triphosphate (ddCTP) to human monocyte/macrophages. Cells of the monocyte/macrophage lineage are an important reservoir of HIV-1. A mouse monoclonal antibody IgG2a was labelled with 125I and modified using N succinimidyl-3-(2-pyridyldithio)propionate (SPDP) as a heterobifunctional reagent in order to conjugate with liposomes to produce a covalent bond (thioether). SPDP-modified antibody was incubated with liposomes containing 5 mol% of maleimido phenyl butyrate phosphatidylethanolamine (MPB-PE) at room temperature (21 degrees C) for 24 h. L-Ab were separated from free and aggregated antibodies by centrifugation. L-Ab were characterized by measuring particle size and binding to anti-mouse IgG-sepharose. Ninety five per cent of the liposomal (L-Ab) lipid label was bound to anti-mouse IgG-sepharose, whereas only 7% of plain liposomes were bound, indicating non-specific binding. Uptake of L-Ab was measured in human monocyte/macrophages as a function of time and compared with that of plain liposomes. The uptake increased with time and it was 4-6 times greater than that of plain liposomes although part of that effect may have been due to unreacted MPB groups.