Larval serum protein 1 is the major haemolymph protein just before puparium formation in Drosophila melanogaster. It has been purified and characterised as a family of hexamers (molecular weight 450000-480000) of three immunologically related polypeptides (molecular weight 75000-81000). Homologous proteins, present in other dipterans, are similar in structure, amino acid composition and developmental profile.