This personal account traces a series of studies that led from DNA physical chemistry to anticancer drug mechanisms. Chemical crosslinking as a basis for anticancer drug actions had been suspected since the time of the first clinical reports of the effectiveness of nitrogen mustard in 1946. After the elucidation of the DNA helix-coil transition, several nearly concurrent findings in the early 1960s established the paradigm of DNA interstrand crosslinking. The DNA filter elution phenomenon was discovered in the early 1970s, and lent itself to the development of practical assays for DNA crosslinks and other DNA lesions in mammalian cells. The assays allowed studies of the effects of DNA damaging agents at pharmacologically or toxicologically relevant doses, and have been widely applied in studies of mutagenic and chemotherapeutic agents. During the period 1979-1986, DNA filter elution studies led to the paradigm of DNA topoisomerases as targets of anticancer drug action, and this has become one of the most active areas of anticancer drug development.