The aim of the study was to compare various PCNA clones (PC10, 19A2, and 19F4) on frozen and paraffin wax sections of ten human glioblastomas. Standard immunohistochemical methods were used (avidin-biotin-peroxidase technique). Half of the paraffin sections were pretreated in a microwave oven. With few exceptions positive staining was achieved only on paraffin sections after microwave treatment. Staining with the 19A2 antibody revealed few (<1%) positively stained tumor nuclei in only three cases whereas staining with 19F4 was positive in eight cases with a labeling index (LI) in the range of <1-3.5%. All cases showed positive staining with the PC10 antibody with an LI of <1% in three cases and in the range of 2.0-17.2% in the remaining seven. However, most of the PC10 stained sections showed a disturbing unspecific background staining. Thus, in spite of the background staining we conclude that PC10 offered the best immunostaining of the antibodies tested.