We studied tumour Apparent Diffusion Coefficient (ADC), necrosis and cellularity in untreated rodent tumours of twelve types from six centres using two different vendors’ equipment. Tumour types included human xenografts (conventional and patient-derived) genetically engineered mouse tumours and syngeneic rat tumours. Across this broad spectrum there was a robust inverse correlation between ADC and cellularity, and a weaker positive correlation between ADC and necrosis. ADC mean and median were the best correlates for cellularity, while ADC standard deviation and kurtosis provided the best correlates for necrosis. This work advances the biological validation of ADC as a biomarker of tumour cellularity.