cDNAs encoding the mouse insulin-like growth factor II (IGF-II) precursor have been isolated from a placental library and sequenced. Mouse prepro-IGF-II is predicted to be 180 amino acids and has 84% and 97% identity with human and rat IGF-II precursors, respectively. There are two prepro-IGF-II transcripts in mouse placenta; the major transcript is 3850 bases and the minor one is 4800 bases. Prepro-IGF-II mRNA cannot be detected in adult mouse liver, but it is readily detected in adult human liver, indicating that there are differences in the regulation of expression of the IGF-II gene in the liver of adult mice and human beings.