Roskilde University Centre, the University of South Denmark and the Panum Institute - Copenhagen University's faculty of health - are among Danish universities tightening security following threats from an anonymous group that took responsibility for a fire at Århus University.
The group said it would fight the "evil tinkering with life" of gene technology, cloning, transplants and artificial insemination. It has declared "a just war" against people involved in "this historic crime".
Ebbe Traerup, of the Panum Institute, said: "We can't prevent people trying to blow us up simply by tightening security, so we've made a point of informing the staff how they should act in an emergency."
The institute has equipment valued at more than DKK100 million (£8 million) but the main fear is that an attack could harm the 6,000 or so people passing through its doors every day.
The universities are also concerned that years of research could go up in flames.
Last month, arson at Århus University caused damage amounting to almost DKK6 million.
The fire, at the department of microbial ecology, and the threats have surprised department staff because they do no conduct genetic research.
Detective inspector Allan Lund of Århus CID, which is investigating the case, said that the letters were written in the style of old books, making it difficult to ascertain the precise meaning.
The group has indicated religious connections. Otherwise, no clear picture has emerged of who may be responsible or whether the letter writers are connected with animal-rights group, anti-abortionists or other groups in Denmark or elsewhere.
Detective superintendent Bjarne Laursen of Århus CID said: "The people who sent these letters have taken pains to write a lengthy explanation of their motives. They're not a group of amateurs."