Enzyme distribution in mammalian membranes**The following abbreviations are used: ER, endoplasmic reticulum (excluding the Golgi); PM, plasma membrane (the equivalent term ‘cell membrane’ is not favoured). The usage of the terms ‘cytochemical’ (here restricted to EM studies), ‘microsome’ and ‘microsomal [or mitochondrial, lysosomal, etc.] fraction’ is indicated in the text. Membranes in apposition are best termed ‘paired', not ‘double’. A ‘vesicle’ is a ‘small bladder-like vessel’ or ‘sac’ (Oxford Dictionary). The centrifugal procedure commonly termed ‘differential centrifugation’ is here termed ‘differential pelleting’, in contrast with ‘banding’ or ‘zonal centrifugation’; the latter term does not necessarily imply use of a zonal rotor (Reid, 1972a; Reid and Williamson, 1974).