A distinctive type of speckled antinuclear antibody staining pattern was identified by indirect immunofluorescence on mouse kidney substrate in 4.8% of 5,976 specimens analyzed for antinuclear antibodies. This pattern, termed variable large speckles (VLS), consisted of 3-10 nuclear speckles ranging in size from approximately 0.2-2.0 mu. The pattern could be differentiated from other indirect immunofluorescence patterns related to specific antibodies. The predominant immunoglobulin isotype demonstrating the VLS pattern was IgM in 27 of 28 sera examined and IgG in 1 serum. VLS sera had substantial IgM antibodies to histone demonstrated by enzyme immunoassay, and further analysis of representative sera showed predominant antibody activity to histone class 3 (H-3). Adsorption with histone H-3 resulted in decrease or removal of antibody producing the VLS pattern. Available information showed that most patients with IgM antibodies of the VLS pattern had undifferentiated connective tissue disease symptoms. They were characterized by a heterogeneity of chronic symptoms including arthralgias, myalgias, inflammatory polyarthritis, myositis, sicca symptoms, and pleurisy associated with elevation of the erythrocyte sedimentation rate. It remains to be determined whether the IgM anti-histone H-3 profile of these patients is a transient or long-standing serologic characteristic.