Study sets have been prepared on mouse hematopoietic neoplasms, mouse ovarian tumors and lesions induced by polyoma virus in mice. Completion of a study of 600 cases of lymphomas in NSF/N mice, congenic for retrovirus genes with high expression, has established the following: A) marginal lymphomas occur frequently (a finding not previously established), and they are not uncommonly juxtaposed with follicular lymphomas; B) a comparison of biopsy and autopsy observations showed that mouse lymphomas progress fairly rapidly from low to high grades; C) tumor progression is associated with clonal selection; D) B-cell lymphomas are almost entirely comprised of cells with an Igm, lk, d- phenotype; E) B-cell lymphomas are transplantable in syngeneic recipients; and F) new retroviral integration sites are usually seen in lymphomas. Lymphomas from other mouse strains have also been studied. Mice of the NZB strain also showed a high incidence of marginal zone lymphomas. C3H and BALB/c mice with gld and lpr mutations associated with dysfunctional Fas and FasL genes, developed plasmacytomas. Additional studies of rat tumors induced by nitrosamines, particularly nasal tumors, are in progress. With the extensive help of Dr. J.C. Eastep, Department of Veterinary Pathology, AFIP, the entire database for the Registry has been placed in the ACCESS database software program to greatly facilitate data storage. Plans have been made to relocate the Registry to a suitable site which will provide continued access and permit utilization by the scientific community.