Using immunological techniques urinary hCG, pregnancy-specific beta 1-glycoprotein (SP1) and beta-subunit of hCG (beta-hCG) levels were measured in the serum and cerebrospinal fluid of patients with trophoblastic tumours. After removal of hydatidiform moles, urinary hCG, beta-hCG and SP1 levels were similar, but SP1 levels tended to exceed serum beta-hCG levels. SP1 usually disappeared first. In patients with metastatic choriocarcinoma, SP1 levels appeared to be lower than beta-hCG values in serum and cerebrospinal fluid, but urinary hCG, beta-hCG and SP1 concentrations all decreased during chemotherapy. Measurements of SP1 levels may well be useful in the monitoring of patients being treated for trophoblastic tumours.