Serum activities of bone alkaline phosphatase (b-ALP) and of tartrate resistant acid phosphatase (tr-ACP) were evaluated in 271 cancer patients; 120 of them had bone metastases (BM) and 151 had none. Correlation coefficients, specificities, sensitivities, negative and positive predicting values were computed. They showed the important contribution that these isoenzymes can bring to the diagnosis of BM in 80 patients with prostate cancer, and to the followup of 191 patients with breast cancer. The assay results were analysed in parallel with bone scan and radiography. They were also compared to those of serum antigens: PSA and PAP for prostate cancer, and CEA and CA15.3 for breast cancer. These results clearly indicate that both isoenzymes are better correlated with BM than antigens, these antigens being markers of the whole tumor burden--primary tumor, metastases, recurrence--whereas b-ALP and tr-ACP are specific markers of bone metabolism.