"I had previously said in my writings that the activity of the tissue researchers who produced WI-38 and MRC-5 was wrong because it constituted immediate material cooperation in the intrinsically evil action of abortion. A more detailed review of the evidence suggests that the tissue researchers played a much more direct role in the culture of abortion than I had realized. Hence, I revise my view to say that those tissue researchers were engaged in immoral formal cooperation with abortion. The activity of those who established these cell strains should be distinguished from that of the researchers who used them to invent the new vaccines. The latter, I continue to hold, were engaged at the level of immoral proximate material cooperation."