Steady-state room-temperature emission spectra have been determined for spinach and barley (both wild-type and the chlorina mutant lacking Chl b and LHCII) thylakoids, a spinach Photosystem II membrane preparation and Chlorella cells, under conditions in which the Photosystem II reaction centres were either open or closed. The data show that the fluorescence quenching efficiency of open reaction centres is greatest for those chlorophyll species emitting in the 680–690 nm wavelength range. The reaction centre quenching efficiency decreases at both shorter and longer wavelengths. Calculation of the fluorescence emission ratio spectra for membrane preparations containing LHCII (wild-type barley thylakoids and isolated LHCII) with respect to those without LHCII (chlorina barley mutant thylakoids) indicates that this complex has characteristic emissions around 650 nm (Chl b) and 680 nm. The Chl b emission is about 2% that of the main Chl a emission at 683 nm (open reaction centres). As the characteristic LHCII emission bands near 650 nm and 680 nm are almost absent in the ratio between LHCII emission spectrum and both spinach and wild-type barley thylakoid emission spectra at open reaction centres, it is concluded that most Photosystem II fluorescence is from LHCII when reaction centres are open. Thus, excitation energy does not seem to be preferentially localised in the core antenna complexes prior to fluorescence emission. The data are interpreted in terms of a model in which energy transfer from antenna to reaction centres is basically diffusion limited.