A study comparing the usage of monoclonal and polyclonal antibodies specific for immunoglobulin light and heavy chains was performed on frozen-tissue sections of 30 B-cell non-Hodgkin's lymphomas. In 16 cases, monotypic staining for an immunoglobulin light chain was demonstrated with monoclonal antibodies using a three-step avidin-biotin peroxidase complex (ABC) method; 13 cases were positive for kappa. In 14 cases, no immunoglobulin light-chain production was demonstrated. Repeat staining of 11 of these 14 cases with polyclonal anti-sera by a direct immunoperoxidase method demonstrated monotypic staining for light chain in 10 cases, 9 of which were positive for lambda. In 22 of 30 non-Hodgkin's lymphomas, an immunoglobulin heavy chain was identified using monoclonal anti-sera. In eight cases, however, no heavy chain was found. Repeated staining with polyclonal sera of additional sections in three of eight cases demonstrated heavy-chain production in each case. Decreased sensitivity, especially for the detection of the lambda light chain, rendered this particular lot of monoclonal antibodies unsuitable for immunophenotyping non-Hodgkin's lymphomas. Variability of antigenic sites on the immunoglobulin molecule seems a likely explanation for these observations.