On 296 blood samples obtained from 22 bone marrow and 21 kidney transplanted patients, the concomitant measurements of polyclonal and monoclonal cyclosporine (CsA) were performed and the relative polyclonal/monoclonal (P/M) ratios were calculated. Biochemical profiles of kidney and liver functions were also determined in all patients. For each type of transplant, biochemical data were divided into two subgroups on the basis of P/M ratio: A) data obtained in patients with P/M ratio > 3.0; B) data obtained in patients with P/M ratio < or = 3.0. While in kidney transplanted patients no difference of biochemical profiles was found between two subgroups, in bone marrow transplant recipients the subgroup A showed a worsening of hepatic function parameters as compared to subgroup B. Therefore, it appears that P/M ratio could represent in bone marrow transplantation an index of hepatic CsA toxicity.