Bone-specific alkaline phosphatase in plasma as tumour marker for osteosarcoma. The levels of bone-specific alkaline phosphatase (BALP) in plasma and tumour tissue samples of 20 Chinese patients with osteosarcoma in Hong Kong were measured by the wheat germ lectin precipitation technique. The plasma BALP levels in these patients were significantly higher than those of the normal subjects (p < 0.001), and also significantly higher than those of patients with benign bone tumor and those of patients with malignant tumor metastasized to the bone (p < 0.0001). Considering the prognostic value of BALP for osteosarcoma, the plasma BALP levels at the time of diagnosis were found to be significantly related to the rate of disease recurrence (p < 0.05). Furthermore, at the time of relapse, the plasma BALP levels in the group of recurrent osteosarcoma patients were significantly higher than those of osteosarcoma patients showing no recurrence (p < 0.05). When ALP was assayed in the tumor tissue, the BALP levels were also significantly higher than those of the control cortical bone extracts in the same group of patients (p < 0.05). We conclude that plasma BALP is a sensitive and specific biochemical parameter in the diagnosis and the subsequent monitoring of osteosarcoma.