The staining patterns obtained with two antibodies against proliferating cell nuclear antigen (PC10 and 19A2) and another cell cycle associated antibody (KiS1) were compared with each other and with a number of established prognostic markers of breast carcinoma. Although PC10 and 19A2 staining patterns were similar, only the latter was significantly associated with KiS1 antibody staining. These findings suggest that the two PCNA antibodies detect different epitopes. KiS1 was the only antibody to show an association with S phase fraction measured by flow cytometry (p < 0.001). It was also associated with histological grade (p = 0.003), oestrogen receptors (p = 0.045), and DNA index (p = 0.007). PC10 showed no association with any of the markers of prognosis, while 19A2 was associated with histological grade (p = 0.017) and oestrogen receptors (p = 0.043). The two PCNA antibodies do not seem to be of value in measuring proliferative activity nor do they seem to be associated with established markers of prognosis in breast cancer.