This study was performed to evaluate the role of protein kinase C (PKC) activity in the development of chemoresistance in clinical breast cancer cells. To simulate the clinical situation, native tumor cells derived from 10 patients with advanced breast cancer were brought into short-term cultures, and treated with anthracyclines (doxorubicin, mitoxantrone), paclitaxel and combinations, respectively. After 3 days of incubation, we determined total PKC activity relative to each control incubated with blank medium. Furthermore, we determined the chemoresistance against these drugs from each cell population separately. Relative PKC activity ranged from 14 to 249%; 64% (37 of 58) of the breast cancer cell suspensions were considered chemoresistant. There was a non-significant trend to a higher relative PKC activity in resistant cells compared to non-resistant cells (p=0.058), regardless of the antineoplastic agent investigated. The individual variability in both PKC activity and chemoresistance pattern revealed that dysregulated PKC activity mediates resistance to antineoplastics. In order to achieve clinical value, evaluation of more data concerning the PKC signal-transduction pathway is necessary. New protocols of cancer treatment will require this information in order to be successful.