Walter J. Grundhauser was a research assistant the University of Chicago's Metallurgical Laboratory ("Met Lab") during the Manhattan Project.
At the age of twenty-four, Grundhauser was recruited the join the Met Lab. At the time, he was a graduate fellow at St. Louis University School of Medicine. Although the information about the job was vague, the St. Louis Post-Dispatch reported that "Grundhauser had an inkling that the position had to do with radiation when he recognized one of the interviewers as the author of a paper on alpha particles."
At the Met Lab, he worked in the Health Division. As a biophysicist, his focus was determining the short-term biological effects of full-body radiation and radioisotopes on mice, rats, and rabbits.
Grundhauser worked at the Met Lab from 1943 until 1945. In April of 1945, he was advised to take a month's leave because he was suffering effects from being overexposed to radiation.
Grundhauser told the Post-Dispatch that when signing onto the job, he and his colleagues were promised that the bomb would be only used for defense. Grundhauser made his position clear when he became one of seventy scientists to sign the Szilard Petition, a document written by physicist Leo Szilard petitioning President Truman to avoid using the atomic bombs on Japan.