International Development Minister Grant Shapps quit Saturday, saying "the buck should stop with me."
Shapps was party chairman until earlier this year, and has been accused of failing to act on claims of bullying and abusive behavior in the party's youth wing.
Conservative activist Eliott Johnson, who was 21, was found dead in an apparent suicide in September. He had accused Conservative youth organizer Mark Clarke of bullying.
Clarke denies wrongdoing but has been expelled from the party.