Rapamycin (RAPA) is a potent new immunosuppressive drug. Although blood concentration monitoring of RAPA is being performed in preclinical and clinical trials, little is known regarding the blood distribution of the drug. Such information would have an impact on the medium used for analysis of the drug. The distribution of RAPA was investigated by spiking human whole blood having an initial temperature of either 4 degrees C or 22 degrees C with a constant amount of 3H-RAPA and increasing amounts of RAPA to a final concentration of 5-100 micrograms/L. The drug concentration spans the range seen when immunosuppressive doses of the drug are administered. This was followed by incubation of the blood at 37 degrees C for 0 to 60 min before separation of cells. The dpm in the resulting plasma and RBC fractions was determined by scintillation counting. The plasma to formed blood elements and plasma to whole blood ratios were 0.05 +/- 0.051 and 0.09 +/- 0.016, respectively (mean +/- SD, n = 50). The distribution did not exhibit any temperature or concentration dependence. The proportion of the drug among cellular components was as follows (mean % distribution +/- SD); RBC 94.5 +/- 4.9%; plasma 3.1 +/- 2.5%; lymphocytes 1.01 +/- 1.02%; and granulocytes 1.0 +/- 0.88%. The free or unbound fraction of RAPA over the plasma concentration range of 5-100 micrograms/L as determined by ultracentrifugation was 2.5 +/- 0.2%. The drug was found to be associated primarily with nonlipoprotein fractions in plasma. The results suggest from an analytical perspective that whole blood as compared with plasma would be the most suitable medium for analysis due to the higher concentrations found in the former.