The cytostatic drug daunorubicin exerts its toxic action by intercalating into the DNA. The efficacy of daunorubicin depends on the intracellular amount in the tumor cell. Here we have evaluated the use of a multiwell-multilabel reader for the direct determination of the fluorescent cytostatic drug daunorubicin in a prostate carcinoma cell line (AT1 R-3327 Dunning prostate carcinoma cells) grown on 24-well plates. We present evidence that this simple fluorescent parameter is a good measure for the toxicologically relevant amount of the drug intercalated into the DNA and, therefore, is a good predictor for the drug's cytotoxicity. The amount of cationic cytostatics in a tumor cell is primarily a function of the efflux pump protein p-gycoprotein (pGP). Therefore, it is of great value that the assay is also suitable for the estimation of the multidrug resistance efflux pump (pGP) activity.