Management of heart transplant patients who are being given cyclosporine is complicated by the variety of methods available for measuring cyclosporine levels and the current trend toward exclusive use of monoclonal assays. To facilitate clinical decisions regarding cyclosporine levels when converting from a polyclonal system to monoclonal system, 79 heart transplant patients underwent a prospective study to compare the polyclonal radioimmunoassay with the monoclonal-specific parent compound and nonspecific radioimmunoassays. Multivariable regression analyses were performed to generate best-fit equations for conversion of one assay to another. The closest correlation was noted in converting the measurement of cyclosporine parent compound with metabolites by the polyclonal method to the monoclonal method (nonspecific), R2 = 0.93. Considerable variability existed in the relationship between polyclonal and monoclonal-specific levels (R2 = 0.66) and between monoclonal-nonspecific and monoclonal-specific levels (R2 = 0.66), and both relationships were affected by hepatic function. Inferences: (1) The conversion of numeric cyclosporine levels from parent compound to parent plus metabolites is not completely predictable and must be empirically determined with the generation of appropriate regression equations. (2) Caution is advisable when a transplant team adopts a new cyclosporine assay for clinical use; formal study between existing assay methods and any newly adopted assay is warranted to prevent inadvertent underdosing or overdosing with cyclosporine.