Small nuclear ribonucleoprotein particles (snRNPs) were identified in nuclear sonicates of Novikoff hepatoma ascites cells and in intact Novikoff hepatoma and PtK2 cells by immunofluorescence and immunoelectron microscopy. Auto-antibodies (anti-Sm and anti-RNP) obtained from patients with systemic lupus erythematosus an autoimmune disease, were used to localize snRNP particles. The Sm antibody is specific for U1, U2, U4, U5 and U6 containing snRNPs. The RNP antibody is specific for only U1 containing snRNPs. Isolated particles, 120 +/- 10 A in diameter, were found to be associated with ferritin-conjugated goat anti-human antibodies coupled to Sm antibodies. In addition, these particles (snRNPs) were occasionally associated with larger particles measuring 230 +/- 10 A in diameter which are presumed to be hnRNP particles. Double label immunofluorescence and immunoelectron microscopy have shown Sm and RNP antibodies to colocalize in PtK2 cells. However, the perinucleolar chromatin and juxtanuclear envelope chromatin was devoid of RNP immunostaining. Therefore, U1 containing snRNPs do not appear to be in these regions. The Sm antibody localizes in a nuclear network including the perinucleolar chromatin and juxtanuclear envelope chromatin. Cells treated with the drug DRB (5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole), which inhibits hnRNA synthesis, show an altered pattern of Sm immunostaining. Such cells contain large clusters of snRNPs which do not extend to the perinucleolar chromatin or perinuclear lamina chromatin. Nuclear matrix preparations maintain an snRNP nuclear network as visualized by Sm immunofluorescence. It is notable that the size and density of the immunostained particles in the nuclear network during interphase, is similar to that of interchromatinic granules.