A new mutation at the locus encoding the mast cell growth factor (Mgf) is described and designated as MgfSl-3Neu. Homozygous mutants have a light grey fur, sometimes with white patches. Homozygotes are fertile, but with reduced litter size, when mated inter se. Analysis of haematological parameters indicated no difference between mutant and wild-type mice. Sequence analysis of the cDNA obtained from the brain of homozygous mutants revealed an A-->G exchange at position 400 leading to a predicted amino acid exchange from Asn-->Leu at position 122. As a consequence of the predicted amino acid exchange an extension of the alpha-helical context and a decreased hydropathicity of the region at positions 101-125 can be deduced. This single amino acid exchange is outside of the known important domains of MGF and explains the weak phenotype of MgfSl-3Neu.