In this paper, we summarize our five-year observation of two-dimensional polyacrylamide gel electrophoretic (2-D PAGE) analysis of immunoglobulin (Ig) light (L) chain patterns on serum/plasma and/or purified human Ig, and compare this technique with agarose electrophoresis and/or immunofixation examination. Polyclonal Ig L chains were seen as large "fuzzy" areas with several zones of high density. The majority (71%) of the monoclonal Ig L chains of monoclonal gammopathy detected by conventional electrophoresis appeared as a single large and well-defined spot on 2-D PAGE analysis, with the remaining appearing as multiple spots. The presence of oligoclonal Ig, reflected by multiple spots in 2-D PAGE, and several bands in immunofixation, was observed in 5 of 26 patients after allogeneic bone marrow transplantation and in 5 patients with tumors. In the majority (77%) of hypergammaglobulinemia, L chains appeared as a wide spread of small and well-defined spots in 2-D PAGE analysis. This pattern suggested oligoclonal Ig-secreting B cell clone expansion, and corresponding abnormalities were not detected with immunofixation. 2-D PAGE analysis also detected oligoclonal Ig expansions whereas conventional electrophoretic examination was normal in 10 additional patients after bone marrow transplantation, and in 6 of 10 immunocompetent patients with acute severe infections. Analysis of the Ig L chain pattern of a severe combined immune-deficient mouse populated with human peripheral blood leukocytes confirmed the skewed human Ig production in the model. In summary, 2-D PAGE appears to be a sensitive tool for the analysis of Ig diversity in various clinical situations.