Ceruloplasmin levels in sera from human patients with osteosarcoma. Ceruloplasmin (Cp) levels in sera of osteosarcoma patients, women on chemical contraceptives, and normal individuals were determined by radial immuno-diffusion (RID) and rocket electroimmunophoresis (REP) techniques; serum copper levels (SCL) were determined by atomic absorption spectroscopy. Ratios of SCL/Cp were constant in all groups tested, indicating that the ratio of bound to unbound copper in the serum is maintained relative to normal levels both in osteosarcoma patients and in women receiving chemical contraceptives. There was a highly significant increase over normal levels in Cp concentration in sera from patients with osteosarcoma and from women on chemical contraceptives, in agreement with our previous data on SCL. Either RID or REP can be used to measure serum Cp. The latter method appears to be faster and more sensitive, whereas the former is easier to perform. It appears that the determination of serum Cp in osteosarcoma patients may be of value in differential diagnosis of the disease in selected patients, and in evaluation of the efficacy of therapy.