This study evaluated the clinical utility of a commercially available chemosensitivity assay. In the first part of the study, tumor tissues from dogs with various malignancies were tested, and the dogs were treated with a mitoxantrone/cyclophosphamide combination protocol. Tumor response was evaluated and compared to the predicted response. Assay results were not a significant predictor of clinical response to chemotherapy or of survival time. In the second part of the study, assay results were used to direct therapy in dogs with refractory lymphoma. There was no significant correlation (p equals 0.323) between predicted response and case outcome.