Quantitation of immunohistochemical staining of the proliferating cell nuclear antigen (PCNA, clone PC10) by image analysis was performed on benign and malignant mammary tumours of dogs and cats. Scoring of the slides was carried out by image analysis to assess the percentage of labelled nuclei (expressed as a ratio of areas). Either the strongly labelled nuclei (SP-PCNA index), or all of the stained nuclei (TP-PCNA index) were counted as positive to determine the growth fraction and its correlation with the histopathological classification and nuclear grade (degree of nuclear differentiation, considered a morphological correlate of tumour aggressiveness). A significant difference in the values of PCNA indices was seen between benign and malignant growths (P < 0.0001, dog; P < 0.05, cat). Neither of the PCNA indices showed correlation with nuclear grade in dogs (P = 0.14 for SP-PCNA index and P = 0.31 for TP-PCNA index) or cats (P = 0.09 for SP-PCNA index and P = 0.07 for TP-PCNA index). A significant difference in the number of mitoses, expressed as mitotic index, was seen between benign and malignant growths in the dog (P < 0.01) but not in the cat (P = 0.078). Good correlation of mitotic index with nuclear grade was revealed in canine malignant growths (P < 0.05), but in feline malignant tumours such correlation (P < 0.05) was shown only when the values of intermediate plus typical forms were compared with the data for atypical forms. It is concluded that quantitation of PCNA-positive nuclear area by image analysis provides an objective method for assessing proliferative activity in benign and malignant mammary tumours of dogs and cats.