Volunteer's blood (1.0 ml/condition) was incubated at 37°C for 15 min with indicated concentrations of WR-1065 and/or mesna. At the end of incubation, mBBr was added to a final concentration of 30 mM, and the derivatization was allowed to continue in the dark at RT for 30 min. The plasma was recovered by centrifugation, and the RBC pellets were washed with 0.9% NaCl. The plasma and blood cell acid-precipitated pellets were washed twice with normal saline and allowed to dry to completion at RT. The weights of the dry pellets were determined, and the pellets were suspended in 100 mM Tris-Cl, pH 8.0, and analyzed as described under Materials and Methods.