We have previously developed a daunorubicin resistant subline of Ehrlich ascites carcinoma (EA/DR) for studies on the reversal of daunorubicin resistance. The mean survival of untreated BALB/c mice bearing drug sensitive parental tumour (EA/DS) is 18.4 +/- 0.6 days, mice bearing EA/DS treated with five daily doses of 0.3 mg kg-1 daunorubicin greater than 60 days, and mice bearing EA/DR treated with the same daunorubicin regimen, 21.1 +/- 1.4 days. We now report complete reversal of daunorubicin resistance in EA/DR by cyclosporin A (CsA). The in vitro daunorubicin IC50, defined as that concentration of daunorubicin required to inhibit 50% of DNA synthesis, in EA/DR was 6.7 +/- 1.15 micrograms ml-1 compared to 2.8 +/- 0.72 micrograms ml-1 in EA/DS. This value was reduced to 2.8 +/- 0.52 and 2.1 +/- 0.10 micrograms ml-1 daunorubicin by 3.3 and 13.2 micrograms ml-1 CsA respectively, P less than 0.05. The MST of groups of host mice bearing EA/DR either untreated, treated with five daily doses of 0.3 mg kg-1 daunorubicin, treated with 80 mg kg-1 CsA in five divided daily doses or treated with combined daunorubicin-CsA were 19.0 +/- 1.0, 21.1 +/- 1.4, 24.0 +/- 2.6 and greater than 60 days respectively. The mean survival of groups of host mice bearing EA/DR treated with 5 mg kg-1 or 10 mg kg-1 CsA simultaneously with daunorubicin for five days was also greater than 60 days. These differences are highly significant.