Upon hearing her husbands  friend talking about a secret project at the University of Chicago that hired scientists, she applied and was hired in the Biology Division of the Metallurgical Laboratory. Lathrop, who previously avoided work that involved animal experimentation, was now studying the uptake, retention, distribution, and excretion of radioactive materials in animals. Lathrops assignment in the project was to test the biological effects radiation had on animals. She worked on the Manhattan Project from 1945 to 1946.