Examples of a new class of alkylating agents, selenium mustards, were prepared for study of their chemical kinetic properties and cytotoxicities against human lymphoblastoid CCRF-CEM cells. In a series of para-substituted aryl 2-chloroethyl selenides, a linear free energy relationship between the first-order rate constant, k'nbp and sigma p gave a rho value of -1.3, indicating that formation of a cyclic ethylene selenonium ion is the rate-controlling step for alkylation of 4-(4-nitrobenzyl)pyridine (NBP). Consistent with the ethyleneselenonium ion pathway, rates of solvolyses were extremely sensitive to increasing water content, and a positive correlation was found between reactivity with NBP and nucleophilic selectivity (Swain-Scott s constant). The s constant, which predicts for variation in intracellular product spread, varied from 0.53 up to 0.95, equal to aliphatic nitrogen mustards. Alkylating activities based on extent of NBP alkylation, however, showed relatively low values, 8-23% of that of mechlorethamine, possibly due to hydrolysis occurring by a separate pathway from nucleophilic substitution. Reactivities and nucleophilic selectivities both showed positive correlations with cytotoxicities, suggesting that the rate and extent of alkylation of relatively strong nucleophilic centers mediate the biologic effects of these compounds. Two bifunctional selenium mustards were substantially more cytotoxic than monofunctional aromatic selenides. No additional cytotoxicity due to the selenium atom was observed, with the exception of diselenide (-SeSe-) compounds. Thus, selenium alkylating agents kinetically and biologically resemble classical, mustard-type alkylating agents.