The authors summarize the early studies on the isolation, structure determination, biological effects, and mechanism of action of daunorubicin (DNR). In fact, the relationship between affinity for DNA and biological activity was first recorded for DNR and later confirmed on several DNA analogs, both biosynthetic and semisynthetic. A possible divergence was eventually observed with 4-methoxy-DNR. This compound is as active as DNR on the DNA synthesis but eight times more potent on different experimental tumor systems. In this context, it is worth noting that recent data on the N-acetyl derivatives of DNR and doxorubicin indicated the persistence of some activity on P388 leukemia in spite of their strongly reduced affinity for DNA and cytotoxic activity when compared with the parent compounds.