Based on recommendations of participants at the National Toxicology Program (NTP) workshop "Animal Models for the NTP Cancer Bioassay: Strains and Stocks-Should We Switch?" some modifications to the NTP rodent cancer bioassay were made. The B6C3F1 remains the mouse model Used in the cancer bioassay. The use of multiple strains of mice will be explored through a new branch within the NTP. the Host Susceptibility Branch. Several rat models were evaluated; the Harlan Sprague Dawley rat is currently being used in the NTP studies.