RNA from multiple myeloma peripheral blood mononuclear cells was amplified in single-stage reverse transcriptase polymerase chain reaction (RT-PCR), using primers to the Vh family that characterizes the clonotypic immunoglobulin heavy chain (IgH) variable diversity joining region (VDJ) rearrangement in each patient, and primers to the indicated constant region (strategy C). The products of each PCR reaction were subcloned and sequenced. Sequencing of subclones included the 5′ end of exon 1 for each isotype, confirming the specificity of the reaction. The pattern of sequences for each isotype analysis gives a relative frequency of the clonotypic IgH VDJ representation for nonclinical (IgM, IgD, and IgA) as compared with the clinical isotype (IgG).