What action is appropriate when co-authors admit that the first author committed plagiarism? I'm the executive editor of a PubMed-indexed journal. Last year The journal published a paper with four authors in the byline. Today I   received an e-mail from the first author admitting that she translated large parts of a research paper published in a local language and used it in the paper's introduction and discussion without referring to the author. She claimed that she did not realize at the time that this was unethical. The other authors, when they discovered the problem, asked her to send us an e-mail containing these details and also sent an e-mail asking us to withdraw the paper.

I could not access the paper written in the local language because it was published in a local journal. Accordingly I'm not sure how much of the text was used.

We use a plagiarism detection software but for this paper it did not yield  any results that warranted attention.

Is withdrawal the solution? If so, how can I contact PubMed to do it?
Are there any further steps to be taken concerning the authors?
Retract the article.

That's what editors of reputable journals do when an author spontaneously asks for it for reasons of plagiarism.

What other options were you considering? Adding a note saying "The authors admitted to have plagiarized large portions of this article but since our system did not detect it, we publish it anyway."?

It's the duty of the editorial board to execute the retraction, possibly with the support of the publisher. But surely your journal has a standard procedure to handle retraction...  

It certainly would help to notify PubMed that will publish the retraction notice in lieu of the article. I suspect this is part of the information that your journal is providing to PubMed by default...