Retrovirus has been known to be integrated into host DNA as provirus. In the course of evolution, some of them have been integrated into germ cell DNA and inherited from generation to generation as host genes. Most vertebrates have accumulated tens or hundreds of retroviral sequences. To know the aspect of their integration and expression in human, lambda 4-1 and ERV3 were investigated by using genomic DNA or placental RNA from Japanese origin. Southern-blotting analysis showed more than 30 copies of lambda 4-1 or its related sequences were integrated in human DNA, whereas only one copy of ERV3 was integrated. One of the fragments (2.0 kb, EcoRI) of lambda 4-1 was polymorphic and confirmed to be inherited in Mendelian fashion. The size of ERV3 observed in Japanese after digestion by Pvull was the same as that reported in caucasian suggesting ERV3 might be integrated before ethnic diversion. Transcription of these retroviral genes was investigated in placenta by northern-blotting analysis. 4 species of mRNA; 3.6 kb, 3.0 kb, 2.2 kb, 1.7 kb, were observed by full length of lambda 4-1, whereas 3 species; 9.0 kb, 7.3 kb, 3.0 kb by env of ERV3. The amount of mRNA transcribed in human placenta was well correlated in two endogenous retroviruses (r = 0.66). Furthermore, positive immunofluorescence-staining of placental section by using antibody against lambda 4-1 env protein suggested that env protein might be translated from lambda 4-1 provirus and expressed on human placenta.