The folding behaviour is investigated of some sequences of 36 monomers with different proportions of hydrophobic residues in a three-dimensional lattice based on the Miyazawa-Jeigen interaction matrix. It is found that the sequences with good folding properties are those with an optimal number of hydrophobic residues, neither too many nor too few. The reason for the deterioration of folding properties of sequences out of this range has also been analysed.