The human immunoglobulin VH4 gene family is thought to contain approximately 10 germ-line genes and to exhibit little polymorphism. We report here an analysis of VH4 germ-line genes that were amplified from DNA of two unrelated individuals. Ten unique (non-repetitive) sequences were obtained from individual A and 11 from individual B. Nine of these sequences represent new germ-line genes, and 8/9 exhibit only 89%-96% similarity to genes identified previously. Subsets of VH4 genes displayed distinctive nucleotide motifs that account for most of the differences between them. This observation suggests that diversity in the VH4 gene family arose from the acquisition of blocks of nucleotides, rather than by accumulation of point mutations. These nucleotide blocks could have been acquired by gene conversion or by homologous recombination. All of the VH4 genes have a potential N-linked glycosylation site at Asn 60, and some genes encode a second site at Asn 52. The VH4 gene family is larger and more polymorphic than appreciated previously. Immunoglobulin gene polymorphism may make a significant contribution to hereditary variations in the immune response and to the genetic predisposition to autoimmune diseases.