It is known that vertebrate lysosomes can sequester and concentrate metal ions including copper, mercury (and organic mercurials), lead, zinc, tellurium, iron, nickel, silver and plutonium (Koenig, 1963; Rahman & Lindenbaum, 1964; Scheuer, Thorpe & Marriott, 1967; Vaughan, Bleaney & Williamson, 1967; Verity & Reith, 1967; Barrett & Dingle, 1967; Abraham, Morris & Smith 1967; Slater, 1969; Brun & Brunk, 1970). Allison (1969) states that the accumulation of certain metal ions within lysosomes may labilize the membranes under certain conditions. Furthermore, Goldfischer (1965) suggests that the toxicity of copper may not be fully manifested while it is contained within the lysosomes, but overloading of the storage capacity, or damage to the lysosomal membrane, resulting in liberation of copper into the cytoplasm could lead to cell injury. The haemocytes of Ascidia pygmaea have also been shown to accumulate pentavalent vanadium in a process involving sequestration by lysosomes (Kalk, 1963). It follows that quantitative cytochemical measurements of certain aspects of lysosomal function might permit the detection of responses to toxic materials at concentrations below those to which the whole organism would respond.