Abstract Individual choroid plexuses of human fetal brain at different developmental stages were investigated by an indirect immunoperoxidase technique. The number and distribution of prealbumin-, albumin-, alpha-fetoprotein-, transferrin-, IgG-, IgA-, IgM- and alpha 1-antitrypsin-containing choroid plexus epithelial cells were recorded. The distribution of prealbumin in choroid plexus epithelial cells differed from that of other plasma proteins identified in these cells. Prealbumin was present in more than 90% of the cells at all ages examined in contrast to albumin, alpha-fetoprotein, transferrin and IgG, which were found in less than 40% of the cells, and this proportion declined later in gestation to only a few per cent. The telencephalic plexus exhibited a staining pattern for the proteins, except for prealbumin, which was different from that of the diencephalic and myelencephalic plexuses. Double-staining and staining with polyvalent antiserum indicated that the same epithelial cells showed positive reaction for albumin, alpha-fetoprotein, transferrin and IgG. The results are related to the observation of a high concentration of plasma protein in fetal CSF which may result from transcellular transfer from blood to CSF across the choroid plexus epithelial cells.