Cell kinetic parameters in human tumours were determined by in vivo labelling with iododeoxyuridine (IUdR) followed by flow cytometric analysis of tumour biopsies after staining with a monoclonal antibody against IUdR-DNA. The purpose of this study was to determine the variation in these kinetic parameters from area to area within the same tumour. Each patient received a single i.v. injection of IUdR and the biopsy or operation specimen was taken several hours later. Multiple biopsies were taken or the operation specimen was cut into several pieces. Tumour material was stored in ethanol. Each piece was subsequently processed and stained for analysis separately. The duration of DNA synthesis (Ts), the labelling index (L.I., percent IUdR-labelled cells) and the potential doubling time (Tpot) were determined for each sample. The mean and standard deviation (variation between pieces) for each parameter was calculated for each tumour. The coefficient of variation (C.V.) provided the measure of intratumoural variation. Thirteen tumours were investigated, 6 of which were transitional cell carcinomas of the bladder and 7 of which were squamous cell carcinomas, mostly of the head and neck. Ts values ranged from 4.1 to 17.2 h (mean 9.5 h), L.I. values from 1.6 to 18.6% (mean 9.7%) and Tpot values from 2.3 to 15.1 days (mean 7.2 days). Mean C.V.s for Ts, L.I. and Tpot were 10, 24 and 27%, respectively. Most of the variation in Tpot (calculated from the other two parameters), came from the L.I., with Ts showing much less intratumoural variations. It is concluded that this kinetic method using low IUdR doses can be successfully applied in human tumours and has sufficient accuracy for predictive assay applications in which tumours need to be classified according to their proliferation rates. Further developments are required to distinguish normal and malignant cells flow cytometrically, particularly for diploid tumours.