A new cell kinetic approach is presented from which the duration of the S and G2 + M phases can be estimated. The technique involves an analysis of the spatial distribution of labelled cells in sections or sheets of epithelium (i.e. an analysis of clustered labelled cells). The technique is largely independent of the absolute number of labelled cells and hence is not influenced by factors which affect the absolute number of labelled cells. The technique is described and experimental data from dorsal murine skin are presented. The technique has also been simulated mathematically so that the phase durations and their variances could be estimated. The advantages of the technique are: it is technically simple; it provides at least two independent estimates of the phase durations; unlabelled cells need not be counted (compare with LI or PLM analysis); it is independent of variations in the absolute yield of labelled cells, and it is applicable if the LI is low and the S phase is short (where the PLM technique tends to fail).