Intra-tumoural heterogeneity of proliferation has been assessed by taking multiple biopsies from 30 colorectal cancers. Following in vivo IUDR labelling, dual parameter flow cytometry was used to measure tumour DNA index (DI) and labelling index (LI) and to derive DNA synthesis time (Ts) and potential doubling time (Tpot). Heterogeneity was seen for all parameters under investigation. Overall coefficients of variation (CV) and logarithmic transformation of Ts and Tpot (due to their non-gaussian distributions) indicate that LI (CV 25%) was the most variable parameter. Intra-tumoral heterogeneity in Tpot (lnTpot CV = 22%) was less than inter-individual variation (CV = 63%), suggesting that this variation should not be a limitation to the possible usefulness of this technique as an independent prognostic indicator. Correlations of Tpot values were examined between the shortest, the median and the value for a pooled homogenate sample from a single tumour. Using an homogenate, it was possible to accurately predict classification of tumour Tpot values as being below the median ('fast tumours') in 15 of 19 cases (79%). The data suggest that assaying an homogenate may allow a more rapid analysis of a multiply sampled tumour.