Some effects of DNA-intercalating agents (Ethidium bromide, Proflavin, and Actinomycin D) on transformation in B. subtilis have been investigated. Competence of recipients can be induced or repressed by decreasing or increasing the temperature of incubation from 42℃ to 37℃ or vice versa. The transformability of the recipients incubated at 37℃ was several hundred fold higher than at 42℃. Induced competence was maintained for about 3 hours. The transformation frequency was higher when Mg^(++) concentration was increased, but it was independent with Ca^(++) concentration. EDTA, an ion-chelating agent, drastically inhibited the transformation. The appearance of transformants in the pancreatic DNase-interrupted transformation showed a lag of about 5 minutes. The concentration of intercalating agents, which inhibits the transformation but does not affect significantly the total viable cell numbers, was 5 ㎍/㎖, 10 ㎍/㎖, and 5 ㎍/㎖, for ethidium bromide, proflavin, and actinomycin D, respectively. The bacterial cells were treated with the intercalating agents a different times after the DNA uptake. The effect of inhibition on transformation by the intercalating agents was found to be an inverse linear response during the first 20 minutes. The treatments applied for 20 minutes after the uptake of DNA were ineffective. The inhibition of transformation was dependent upon the length of exposure to intercalating agents, and the maximum effect was attained at about 20 minutes. Ethidium bromide was considerably more effective in inhibiting the yield of transformants when it was added subsequent to the DNA uptake than prior to the uptake. A probable cause of the inhibition was investigated by using the tritiated donor DNA. The DNA molecules reisolated from the ethidium bromide-treated bacteria and those from the untreated bacteria showed basically similar radioactivity. Thus, the intercalating agents seem somehow to interfere with the recombination, such as by abnormal pairing, resulting in very poor transforming activity.