Humans express an array of Mhc genes, while the chicken has an Mhc that is relatively small and compact with fewer expressed genes. Here we ask whether the "minimal essential Mhc" of the chicken is representative for birds. We investigated the RFLP genotypes in 55 great reed warblers Acrocephalus arundinaceus and 10 willow warblers Phylloscopus trochilus to obtain an overview of the number of class II B genes. There were 13-17 bands per individual in the great reed warblers and 25-30 in the willow warblers, and every individual had a unique RFLP genotype. The high number of RFLP bands indicates that both species have a large number of class II B genes although some may be pseudogenes. Seven different class II B sequences were detected in a great reed warbler cDNA library. There was considerable sequence divergence between the cDNA sequences in exon 2 (peptide-binding region, PBR), whereas they were very similar in exon 3. The cDNA sequences were easily alignable to a classical chicken class II B sequence, and balancing selection was acting in the PBR. One of the cDNA sequences had two deletions and is likely nonfunctional. Finally, the polymorphic class I and class II B RFLP fragments seemed to be linked in the five studied great reed warbler families. These and previous results suggest that birds of the order Passeriformes in general have more Mhc class I and II B genes than birds of the order Galliformes. This difference could be caused by their phylogenetic past, and/or by variance in the selection pressure for maintaining a high number of Mhc genes.