Genomes of all mammals encodeapobec3genes, which are thought to have a function in intrinsic cellular immunity to several viruses including human immunodeficiency virus type 1 (HIV-1)1. APOBEC3 (A3) proteins are packaged into virions and inhibit retroviral replication in newly infected cells, at least in part by deaminating cytidines on the negative strand DNA intermediates2. However, the role of A3 in innate resistance to mouse retroviruses is not understood. Here we show that A3 functions during retroviral infectionin vivoand provides partial protection to mice against infection with mouse mammary tumour virus (MMTV). Both mouse A3 and human A3G proteins interacted with the MMTV nucleocapsid in an RNA-dependent fashion and were packaged into virions. In addition, mouse A3-containing and human A3G-containing virions showed a marked decrease in titre. Last, A3−/−mice were more susceptible to MMTV infection, because virus spread was more rapid and extensive than in their wild-type littermates.