The expression of J chain and μ chain genes has been investigated during human fetal ontogeny by the polymerase chain reaction performed on liver sections. With this technique, we have been able to detect expression of J chain and μ chain in a single 6-10-μm-thick formalin-fixed paraffin-embedded section of fetal liver tissue. J chain expression reached a maximum at 16 weeks of gestation but was clearly detectable in the liver at the 6th week. Although not detectable at the 6th week, μchain mRNA became readily detectable at 7 weeks of gestation. These results indicate that J chain expression precedes that of μchain by at least one week. Therefore, our results imply that J chain is the first immunoglobulin-related polypeptide expressed during the embryogenesis and differentiation of B cells in the fetal liver. © 1993, Japan Society for Cell Biology. All rights reserved.