Republican presidential candidate Ben Carson responded late Thursday to what he called "propaganda" regarding his past involvement in research using fetal tissue.

"I have not actually worked with fetal tissue," the former neurosurgeon said on Fox News's "The O'Reilly Factor."

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"To suggest that I'm in the laboratory actually doing the research or retrieving fetal tissue is nothing but propaganda," Carson said later, noting that fetal tissue can "come from a variety of different places."

"Ectopic pregnancies, spontaneous abortions — but just because they get the fetal tissue, does it mean they should throw it out? Of course they don't. That's how science is advanced," Carson said.

Carson came under fire this week after a blog highlighted a 1992 academic paper that lists him as one of four co-authors for research that used brain tissue from fetuses aborted at nine and 17 weeks of gestation.

He has been vocally critical of Planned Parenthood since an anti-abortion group began releasing secretly recorded videos it says show the health organization trying to profit from the sale of tissue from abortions, which it denies.

"I, nor any of the doctors involved with this study, had anything to do with abortion or what Planned Parenthood has been doing," Carson wrote in a Facebook note that echoed his remarks from Fox.

"The fetal tissue that was viewed in this study by others was not collected for this study," he added, pushing back on "an attack" from "the left and the media."

He wrote that his involvement in the study about tumors consisted of "supplying tumors that I had removed from my patients." Those samples were then "compared to other tissue samples under a microscope."

"You have to look at the intent," Carson told The Washington Post earlier Thursday. "To willfully ignore evidence that you have for some ideological reason is wrong. If you’re killing babies and taking the tissue, that’s a very different thing than taking a dead specimen and keeping a record of it."