Retrovirus-like particles have been isolated from normal fetal human plasma and from different embryonic organs collected from late first-trimester fetuses. The majority of the virus-like particles banded at a density region of of 1.19-1.22 g/ml, although lighter particles having a density of 1.15-1.17 g/ml were observed in some fetal tissues. The particles appeared similar to retroviruses when viewed electron-microscopically. They contained reverse transcriptase (RT) which accepted oligo (dG)-poly(C) in Mn+2 over other synthetic template-primers and transcribed heteropolymeric RNAs primed with oligo (dT). The enzyme was partially (40%) inhibited by the antiserum against RT of feline endogenous virus (RD114) and not at all by the antisera against RT of avian myeloblastosis virus (AMV), spleen necrosis virus (SNV) and gibbon ape leukemia virus (GALV). The simultaneous detection test in the presence of actinomycin D revealed that the particles contained high mol. wt. (70 S and 35 S) RNAs. The single-stranded DNA complementary to RNA of the human fetal particle hybridized to DNA obtained from different tissues of human adults, showing that the nucleic acids of the virus-like particles were endogenous. The particles could be isolated only from the embryonic organs during differentiation. This suggests that the retroviral gene expression is correlated with embryonic differentiation. These particles could not be induced and as yet infectivity has not been demonstrated, therefore, they are at present described as retroviral elements, not as retroviruses.