Lysosomes are small intracellular organelles present in most or all cells of animals of widely different evolutionary development. In general their diameter may vary from 0(2 to 0(5 ,tm so that they overlap the dimensions of mitochondria. In the original differential centrifugation studies they were isolated as the 'light mitochondrial' fraction. It is difficult to give an exact size to these organelles because the term 'Iysosome' covers a wide range of structures from the small primary lysosomes budded off from the Golgi apparatus, to secondary lysosomes caused by the fusion of primary lysosomes with endocytotic vacuoles, to complex fusion-structures such as heterolysosomes and autophagic vacuoles. Indeed, de Duve (1969) suggested that lysosomes are only a part of the intracellular vacuolar system. Consequently the term 'Iysosome' includes granules of various sizes, specific densities, and enzymatic content. Possibly these structures have different complements of enzymes in the various tissues of the same animal. The lysosomes of polymorphonuclear cells, the azurophil, and possibly the specific granules (Goldstein, 1974) are peculiar in both their relatively great size and enzymic complement. Seemingly, however, the main cellular content of hydrolytic enzymes, particularly those acting optimally at an acidic pH, is within the lysosomes and is one of the characterising features of these organelles. They may be defined as small intracellular organelles that contain acid hydrolases bounded by a semi-permeable membrane which is said to control the latency of the intralysosomal enzymes and to allow the organelles to behave as osmometers (de Duve, 1959). By electron microscopy they are seen to be bounded by a single membrane (Daems et al., 1969; Schellens et al., 1977). Lysosomes can be seen when living cells are examined by phase-contrast microscopy, when their greater refractility distinguishes them from the smaller mitochondria or chondriosomes. Indeed, it is now apparent that the lipochondria of the older microscopists probably correspond to lysosomes (Lane, 1968; Munro et al., 1964). Such bodies were seen to congregate around the food vacuole in amoebae shortly after the uptake of food. Correspondingly, in phagocytic cells primary lysosomes become attached to endocytic vacuoles and release their hydrolytic enzymes into these vacuoles, digesting the endocytosed matter (Cohn and Fedorko, 1969).