Although astrocytomas, anaplastic astrocytomas and glioblastoma multiforme differ in their clinical courses, histological distinction between these three tumours and between astrocytomas and anaplastic astrocytomas in particular may be unclear on histology alone especially in small biopsies. In the present study, a more objective way of distinguishing between the three types of tumour is sought. Frozen sections from 26 astrocytomas, 26 anaplastic astrocytomas and 38 glioblastomas were stained by an indirect immunoperoxidase technique using the monoclonal antibody Ki-67 which binds to nuclear proteins in the G1, S, G2 and M phases of the cell cycle. The Ki-67 staining was assessed quantitatively through direct observation of the stained sections by the use of a drawing tube attached to a microscope. A minimum of 1000 cells was counted in each case and labelled cells were expressed as a percentage of the total number of cells. These results, when correlated in each case with the histology of the tumour in paraffin sections, showed Ki-67 labelling indices ranging from 0 to 1.9% (mean 0.5% SD +/- 0.54) in astrocytomas, 0.6 to 10.9% (mean 4.1% SD +/- 2.8) in anaplastic astrocytomas and 0.9 to 16.2% (mean 6.4% SD +/- 3.34) in glioblastoma multiforme. The differences in the three types of tumour were statistically significant. The results suggest that Ki-67 staining is a useful addition to the panel of techniques for distinguishing between astrocytomas, anaplastic astrocytomas and glioblastoma multiforme.