Sera from 35 patients with systemic lupus erythematosus were analyzed for the subclass distribution of IgG autoantibodies to saline-soluble intracellular proteins. To assess the response to all Ag, an immunoblot technique was used, and strips were sequentially probed with patient sera, monoclonal anti-subclass sera, and a labeled anti-mouse reagent. The relative proportions of each subclass reactive with a specific Ag was determined semi-quantitatively by densitometric scanning. Overall, all of the IgG subclasses were involved in the autoantibody response, although the frequency of detection was highest for IgG1 and lowest for IgG4. When the subclass responses to different Ag were compared, IgG1 was the major subclass reactive with the Ro, La, and U1 ribonucleoprotein Ag, whereas IgG1 and IgG2 were almost equally represented in the responses to the Sm BB' and D Ag as well as to the ribosomal P proteins. Individual patient sera frequently showed discordance between the dominant subclass reactive against apparently unrelated proteins and even against proteins within the same antigenic particle (e.g., the Sm BB' and D proteins). These observations indicate that there are two major patterns of subclass response to the common lupus autoantigens but that considerable variation between patients and even within the same patient (to different Ag) occurs.