Serum reactivities towards individual U1 snRNP proteins were determined by immunoblotting in 32 patients with mixed connective tissue disease (MCTD). Time persistence of immunoblot profiles and clinical significance of anti-(U1)RNP antibody specificities were also investigated. IgG anti-(U1)RNP antibodies were found in the sera of 29 out of 32 patients (90.6%): 21 (65.6%) reacted with the 70-kD protein, 25 (78.1%) with A, 23 (71.9%) with C and 20 (62.5%) with B/B' proteins. None were reactive with the Sm-D peptide. Seventy kilodalton antibody specificity was strongly associated with a higher antinuclear antibody titre (> 160) and slightly associated with disease activity; anti-B/B' specificity was associated with lymphadenopathy. Anti-A, -C and -B/B' antibodies were negatively associated with systemic lupus erythematosus (SLE) skin rashes. Two types of anti-(U1)RNP blotting patterns were selected: "full spectrum" (53.1% of cases) and a "partially/no reactive" one (46.9%). Such patterns were unchanged over time in 14 out of 16 cases prospectively examined (87.5%), while the pattern shifted from "full spectrum" to "partially/no reactive" in 2 cases (12.5%): in 1 after a prolonged clinical remission (> or = 4 years) and in the other following immunosuppressive therapy. The anti-(U1)RNP antibody immunoblot profile in MCTD patients consisted of various reactivities and remained unchanged over time in most cases. Antibody reactivity against the 70-kD protein represented the major U1 snRNP specificity. The various anti-(U1)RNP specific reactivities demonstrated poor clinical significance within MCTD. Thus, MCTD seems to be characterized by a longstanding serological heterogeneity whose reactivities do not apparently correspond to distinct features within the broad clinical spectrum of MCTD.