Brequinar sodium (BQR) is a novel immunosuppressive drug that inhibits cell proliferation by virtue of its disruption of the de novo pyrimidine biosynthesis. The basis of the immunosuppressive activity of BQR is distinctively different from that of cyclosporine (CsA), and we have recently evaluated in vivo and in vitro the efficacy of the two drugs when used in combination. Subtherapeutic doses of BQR and CsA were tested for their ability to prolong heterotopic cardiac allograft survival in the MHC- and non-MHC-mismatched ACI-->LEW rat strain combination. The graft survival data derived from these experiments were analyzed using the median-effect analysis to establish the immunosuppressive interaction of both drugs. The administration of BQR 3 mg/kg three times weekly or CsA 2.5 mg/kg daily moderately prolonged cardiac allograft survival, with a mean survival of 10 +/- 0.5 and 16 +/- 5.3 days, respectively. The use of the two drugs in combination with the same dose schedule exerted a synergistic effect on graft survival, prolonging the graft function to a mean of 31 +/- 5.7 days. Sera from animals treated with the two drugs displayed, when compared with single treatment groups (BQR 3 mg/kg and CsA 2.5 mg/kg), significantly (P < 0.01) increased in vitro inhibition of lymphocyte proliferation following stimulation with PHA. Finally, a clear correlation between the mean survival time and BQR plasma levels of animals treated with BQR alone or in combination with CsA was seen. Those treatment groups with BQR levels below 2 micrograms/ml (1.5 and 3.0 mg/kg/3x/week) had a mean graft survival of less than 10 days). In contrast, recipients treated with a combination of low doses of BQR and CsA displayed higher drug plasma levels (> 2 micrograms/ml) and longer mean graft survival times. These observations suggest that BQR and CsA may be highly effective when used in combination to prevent organ allograft rejection for clinical transplantation.