Current sample processing (SP) methods for tacrolimus (FK506) immunoassays are mainly based on extraction of drug by organic solvent and divalent metal ions. Although these methods are effective for drug extraction and interference elimination, they suffer from drawbacks including inconvenience for operation, difficulties for automation and potential measurement bias. To overcome these limitations, this study describes a new SP reagent for blood cell lysis and protein denaturation. A TRFIA (time-resolved fluorescence immunoassay) was developed by using this SP reagent for whole blood FK506 quantification. Results show that blood samples could be turned into homogeneous solution after being treated by this SP reagent, and so could be directly applied to immunoassays without centrifugation. The analytical sensitivity of the FK506-TRFIA was 0.57 ng/mL, the within-run and between-run coefficient of variations (CVs) were both less than 10%. The FK506 values of 126 samples obtained by FK506-TRFIA correlated excellently with that obtained by ABBOTT FK506-CMIA (R2 = 0.982). Comparison studies also show that the FK506-TRFIA was highly resistant to endogenous interferences. These results suggest that the present SP method is a more promising chose for FK506 immunoassay, and in the meantime, its simplicity makes the whole-process immunoassay automation more feasible by obviating the necessary for centrifugation.