Heparin derivatives are disclosed that may be used to inhibit formation of metastases. One series of heparin derivatives of the invention are N-acylated with monocarboxylic acids or dicarboxylic acids and have molecular weights from about 1000 Daltons to about 30,000 Daltons. Another series of heparin derivatives of the invention have average molecular weights ranging from about 1000 to 3000 daltons, and are optionally N-acylated with residues from aliphatic monocarboxylic or dicarboxylic acids.