Elliptical galaxies are thought to have formed most of their stars in a rapid burst at high redshift, but an unambiguous example of a primeval elliptical has yet to be discovered. High redshift radio galaxies provide the most promising candidates, but the presence of an active nucleus complicates the analysis of their evolutionary state from optical observations. We have therefore recently embarked on a programme to search for thermal emission from dust in radio galaxies at z > 2 using ground-based sub-millimetre photometry. We have already detected two sources (Dunlop et al. 1994) and expect to detect many more with more sensitive sub-mm arrays. However, for an accurate determination of dust mass it is vital that these sub-mm observations be complemented by ISO photometry at shorter wavelengths to enable us to constrain the temperature distribution of the dust. We thus propose to make ISOPHOT observations of a sample of 10 high-redshift radio galaxies at 90, 160 and 200 microns (corresponding to rest-frame wavelengths of approximately 25, 40 and 50 microns respectively). Our sample has been selected to be representative of the range of properties (e.g. radio luminosity, radio spectral index, radio morphology, optical continuum shape, line luminosity etc) found in high-redshift radio galaxies to date. Most importantly it spans a factor of 1000 in radio power, a fact which will enable us to unambiguously separate the influence of AGN activity from the general underlying cosmological evolution. These observations will provide the first clear picture of the level of star-formation activity in elliptical galaxies at z = 3.