The invention relates to feline IGF-1, to the nucleotide sequence encoding this protein and to the use of IGF-1 as adjuvant for the vaccination of cats, in particular against the feline retroviruses FIV and FeLV. IGF-1 may be used in the form of protein or may be expressed in vivo by a suitable, e.g. viral or plasmid, expression vector. The invention relates to all types of vaccines, namely inactivated, attenuated, sub-unit and recombinant vaccines. The vectors expressing IGF-1 in vivo may also be used in immunity-stimulating compositions.