We have detected weak absorption bands in the near infra-red region of reduced mammalian cytochrome c oxidase, analogous to those that we have recently reported to be present in the bacterial cytochrome o (Ingledew, W.J., Bacon, M. and Rich, P.R. (1992) FEBS Lett. 305, 167-170). The major band is centred at 784 nm and has an m M 1 c m 1 of around 0.1. It is shifted to 760 nm in the carbon monoxide compound and is absent in the reduced cyanide complex. We attribute it to a charge transfer band of ferrohaem a 3 , equivalent to the 'band III' or 'conformational band' of haemoglobin.