Definitive diagnosis of B-cell non-Hodgkin lymphomas often requires demonstration of B-cell monoclonality. Immunohistochemical detection of monotypic immunoglobulin light chain expression, and thereby B-cell monoclonality, may be accomplished readily using fresh cell suspensions or frozen tissue sections. However, immunohistochemical detection of immunoglobulin light chain expression in formalin-fixed, paraffin-embedded tissues is more difficult; with few exceptions, techniques suitable for formalin-fixed, paraffin-embedded tissues are not widely available. This report describes and validates a method for detecting immunoglobulin light chain expression in formalin-fixed, paraffin-embedded tissues using a heat-induced epitope retrieval technique. This method was evaluated in a series of 113 cases of B-cell non-Hodgkin lymphoma, including 73 cases with correlative flow cytometric immunophenotyping data. Monotypic light chain expression was demonstrated in 91 (81%) of 113 cases, including several small core biopsy specimens with extremely limited tissue. Compared with the reference method (flow cytometric immunophenotyping), the specificity of the assay was 100%. Interobserver reproducibility was excellent, with 87% concordance between two independent observers categorizing cases as indeterminate, suggestive or diagnostic of kappa or lambda light chain restriction (Cohen kappa statistic: 0.81). In summary, the described method permits demonstration of immunoglobulin light chain expression in formalin-fixed, paraffin-embedded tissues in approximately 80% of cases of B-cell non-Hodgkin lymphoma with a high degree of specificity and excellent interobserver reproducibility. The assay is sufficiently robust for diagnostic use in small biopsies in which fresh tissue is unavailable.