One of the mechanisms for multidrug resistance (MDR) of tumors is an overexpression of the P-glycoprotein (P-gp). The cytostatic agent daunorubicin was labeled with carbon-11 to probe P-gp with PET. An enzymatic route for the conversion of carminomycin to [4-methoxy-11C]daunorubicin ([4-methoxy-11C]DNR) was investigated, since attempts failed to prepare daunorubicin chemically using [11C]methyl iodide. In the enzymatic synthesis methylation was accomplished by S-adenosyl-L-[methyl- 11C]methionine (SAM), which was synthesized from L-[methyl- 11C]methionine. This methylation is catalyzed by carminomycin-4-O- methyltransferase (CMT). The overall radiochemical yield of [4- methoxy.11C]DNR is 1% (EOB), with a total synthesis time of 75 min. In conclusion, [4-methoxy-11C]DNR can be successfully prepared from carminomycin and [11C]SAM using enzymes.