The molecular forms of human chorionic gonadotropin (hCG) were assessed in parallel serum and urine samples from a choriocarcinoma patient by gel chromatography, isoelectric focusing, and Western blot analysis. There were significant qualitative differences in the molecular forms of hCG between the serum and the urine. The serum hCG had a greater molecular weight and a stronger acidic charge than the urinary hCG. These results may be attributed to differences of sialic acid contents. Affinity chromatography using Ricinus communis agglutinin-conjugated Sepharose showed that the serum hCG was completely sialylated, while the urinary hCG was partially desialylated. In addition to the complete hCG molecule, two kinds of hCGbeta-related fragments were detected in the urine of the patient, but not in the serum. In the Western blot using anti-hCGbeta or anti-hCGbeta-CTP under reducing conditions, the urine presented a new band (CTP') with Mr 23,000 in addition to the beta-subunit, while the serum revealed a single band of the beta-subunit with or without a reducing reagent. The present data revealed striking qualitative differences in the molecular forms of hCG between serum and urine of a patient with choriocarcinoma.