A comparison of the release of different markers during T cell mediated cytotoxicity of mastocytoma (P815) target cells is presented. It is shown that contrary to common assumption $^{51}$Cr is largely held in the cell in small molecular form and only a small proportion is bound to macromolecules. Apart from this, it is shown that the release of cytoplasmic macromolecules (proteins labelled with radioactive leucine or methionine, and RNA labelled with uridine) show essentially the same kinetics (no lag phase and linear with time) as the release of phosphoryl choline, phosphorylated 3-O-methyl-glucose, sucrose and chromium, indicating that the kinetics of release of cell constituents is not related to their molecular size. Rubidium and nicotinamide are special cases in that both the spontaneous release and release in the presence of effector cells is different from the other cytoplasmic markers. Rubidium shows a rapid efflux which is complete within 15 min. This appears to be a result of T cell contact rather than the first stage of lysis. Nicotinamide shows a biphasic release suggesting that about 10% of the label effluxes as a result of T cell contact. The remaining label is released with similar kinetics to chromium. In contrast to these markers the DNA labels (thymidine and IUDR) are released with a lag of about 30 min, suggesting that the nuclear membrane is damaged later than the cytoplasmic membrane. These data are consistent with the hypothesis that while target cell death is causally related to T cell contact, there is no fixed time relation between the two events. Target cell death is accompanied by the sudden release of the cytoplasmic contents.