The revertibility of three spontaneous hypoxanthine phosphoribosyl transferase (HPRT)-deficient V79 cell lines has been determined after exposure to a number of alkylating agents. TG11 and 19 reverted at frequencies ranging from 1 X 10(-5) to 1 X 10(-4) after exposure to doses of ethylmethane sulphonate (EMS) N-methyl-N-nitrosourea (MNU) and N-ethyl-N-nitrosourea (ENU) resulting in surviving fractions between 1.0 and 0.1. Reversion frequencies in TG15 ranged from 10(-7) to 5 x 10(-6) over a similar dose range. The relative efficiencies of different monofunctional alkylating agents in causing reversion of TG11 at equitoxic doses were ENU greater than EMS greater than N-ethyl-N-nitroso-guanidine greater than MNU greater than N-methyl-N-nitrosoguanidine greater than methylmethane sulphonate. Revertant frequencies for all three cell lines were maximal immediately after treatment and declined thereafter at a rate inversely proportional to dose. Such kinetics are explicable if reversion is due to miscoding opposite alkylated guanines. Reversion frequencies after N-butyl-N-nitrosourea exposure were 100-fold lower than after MNU and kinetics of expression of revertant colonies differed. Frequencies were low immediately after treatment, increased between 0 and 24 h then remained at a plateau. Similar kinetics were observed after chlorozotocin and bis-chloroethylnitrosourea exposure. This difference in expression kinetics suggests that reversion in this case is not the result of direct miscoding but of errors in excision repair. TG11, 15 and 19 had low spontaneous mutant frequencies which were either unaffected or only marginally increased by treatment with 5-azacytidine.(ABSTRACT TRUNCATED AT 250 WORDS)