The metabolism-induced binding of 14C-labelled misonidazole (MISO) to hypoxic V79 cells in multicell spheroids has been quantitated using autoradiography. Hypoxia was shown to be the major determinant of the rate of binding. Maximally hypoxic cells bound MISO several times more rapidly than necrotic material in the centre of the spheroids, and up to 50 times more rapidly than well oxygenated cells. The rate of binding to chronically hypoxic cells at the edge of the necrotic centre was 20 times less than to similar cells in other spheroids made maximally hypoxic with N2. This difference is consistent with the greater radio-sensitivity of the chronically hypoxic cells, which is a consequence of their intermediate level of oxygenation. The results indicated that the ability to bind MISO might have considerable potential as a marker for hypoxic cells in tumours. However, some binding patterns cannot be explained by the simplest model of O2 diffusion. It may be necessary to invoke more complex models of O2 diffusion or metabolic gradients within the spheroid which affect the rate of binding.