A series of novel substituted thiochromones and thiochroman-4-ones was synthesized. Compounds were designed as analogues of naphthoquinone and as potential "bioreductive alkylating agents" and were tested for antitumor activity. The lead compound, 3-(chloromethyl)thiochromone 1,1-dioxide (4), inhibited Ehrlich ascites tumor growth by 100% in CF1 male mice at 10 (mg/kg)/day ip. Similarly, 18 of the 29 related compounds demonstrated good activity in this tumor screen. Few definitive structure-activity correlations were evident regarding the nature of the 3-substituent. However, the 2,3 double bond and a sulfone or sulfoxide were required for activity. Four of the compounds synthesized showed marginal but significant activity against P-388 lymphocytic leukemia.