To examine the specificity of the Incstar Cyclo-Trac SP RIA kit, individual blood samples from 28 cardiac allograft patients on cyclosporine A (CsA) therapy were extracted and chromatographed by HPLC. Initially, eluates from a pool of the above samples were collected at regular intervals and measured by RIA to locate possible cross-reacting compounds. Unknown cross-reacting materials were detected in a fraction (UNK) that was collected before elution of CsA. For each patient's sample, fraction UNK and the fraction containing CsA were then collected and measured by RIA. In 9 of 28 samples, cross-reactivity was detected in fraction UNK; range 11 to 36%, mean 22 +/- 7.5%. Cross-reactivity was not apparent in fraction UNK of CsA-free blood samples from normal volunteers.