A major component of nuclear change in concanavalin-A-stimulated bovine lymphocytes is a severalfold increase in interchromatinic volume, which coincides with nuclear swelling and extensive structural remodelling. Large-scale ultrastructural changes in isolated nuclei and nuclear matrices (NM) reflect those occurring within nuclei in situ during mitogenesis. While nonchromatinic nuclear material embedded within nuclease- and salt-extracted whole cells closely resembled in situ interchromatinic matrices, large NM isolated in solution shrank after chromatin was extracted. Numerous perinuclear filaments persisted throughout NM isolation and cytoskeletal proteins were identified in two-dimensional (2-D) gels of such preparations. Taken together these data indicated that the lymphocyte cytoskeleton is likely continuous with the nuclear matrix and could play a role in maintaining nuclear organization. A wide range of lymphocyte NM proteins were resolved in 2-D gels. Significant changes in protein composition coincided with nuclear structural remodelling. Lamin B was prominent at each stage of nuclear development, whereas lamins A and C were only found in stimulated lymphocyte matrices. Lymphoblast NM contained more large basic proteins. Progressively increasing polypeptide complexity of these NM arose by de novo protein synthesis and posttranslational modifications throughout concanavalin A stimulation. NM from stimulated lymphocytes also contained more ribonucleoproteins, possibly indicating the presence of significant amounts of transcriptional material.