The physicochemical principle of "die and coin" complementarity proffered by Pauling and Delbruck and exemplified in Watson and Crick DNA was used to design new antineoplastic compounds. In search of an explanation for why certain molecules and not others are present in nature, biologically active small molecules were discovered to exhibit complementarity when inserted into cavities between base pairs in DNA. Ligands in the steroid/thyroid hormone/vitamin D family fit particularly well into the site 5'-dTdG-3'.5'-dCdA-3'. Degree of fit of various candidate compounds in the manner of a given hormone correlated with degree of hormonal activity. Hormone antagonists fit into the same site but in a different manner than the agonists. Computer graphics and energy calculations confirmed salient observations including the remarkable complementarity of estradiol and DNA. Using the above criteria, a new candidate antiestrogen, para-hydroxyphenyl-acetylamino-2,6-piperidinedione was successfully designed. Taken as a whole, these results coupled with recent independent findings raise the possibility that the mode of action of certain hormones and hormone antagonists may involve direct insertion into DNA mediated by classical protein receptors and other transcription factors.