Phagosome-lysosome fusion plays an important role in the destruction of micro-organisms by phagocytic cells. Several approaches have been used to study the determinants of phagosome-lysosome fusion in mouse macrophages. Fusion can be assayed by electron microscopy using horseradish peroxidase or thorium dioxide as a marker for secondary lysosomes or by a rapid and quantitative fluorescence assay. In the second method, secondary lysosomes are labelled with the fluorescent vital dye Acridine Orange and the rate and extent of their fusion with yeast-containing phagosomes is monitored by fluorescence microscopy (Hart & Young, 1975; Kielian & Cohn, 1980). Good agreement was found with the results obtained from both methods. The rate of fusion is not affected by the number of particles ingested, by enzymic modification or by concanavalin A cross-linking of the plasma membrane or by coating the phagocytic particle with concanavalin A or immune serum (Kielian & Cohn, 1980, 1981~). Fusion is also independent of drugs that affect the cytoskeleton, such as colchicine and cytochalasin B (Kielian & Cohn, 198 la). A limited number of factors can modulate the fusion process either positively or negatively. The rate and extent of fusion is dramatically increased after several days of cell culture. In 4-day cells, the initial rate of fusion is about eightfold higher than at 5 h (Kielian & Cohn, 1980). Fusion is also highly increased in macrophages activated in vivo or treated with the tumour promoter phorbol myristate acetate (PMA) at concentrations of 0.1-1 .Opg/ml (Kielian & Cohn, 1981b). Macrophages require 2-3 h of pretreatment with PMA to express maximal phagosome-lysosome fusion, and the effect of PMA stimulation is maintained for at least 20 h when cells are returned to PMA-free medium. The protein synthesis inhibitors puromycin and cycloheximide block the enhancement of phagosome-lysosome fusion, suggesting that PMA-stimulated phagosomelysososme fusion requires protein synthesis. The high fusion rate of 4-day cells is not increased by PMA and is unaffected by protein synthesis inhibitors. Fusion is inhibited by decreased temperature (Kielian & Cohn, 1980). No detectable fusion occurs below 15°C and this inhibition is rapidly reversed when cells are returned to 37°C. Fusion is also inhibited by lysosomal uptake of a number of polyanionic compounds containing high densities of sulphate, sulphonate or carboxylate residues (Hart & Young, 1980). Dextran sulphate (DS) in pg/ml quantities is an excellent inhibitor, whereas nonsulphated dextran is without effect at 1000-fold higher concentrations (Kielian et al., 1982). The DS inhibition correlates with its accumulation in high concentration