The distribution of the calcium-binding protein calbindin D-28K (CB) was investigated in human fetal primary visual cortex. CB is present in Cajal-Retzius cells of layer I, in sparse neurons of the ventricular and intermediate zones (VZ, IZ), and in tangential fibres in IZ by 15 weeks (W) of gestation. Cajal-Retzius cells lose their staining by 30W. CB appears in layers II-VI mainly from 26W, following an inside-outside sequence. Until 34W, CB labelling is in somata and neuropil located primarily in layers IVA, IVC and V. Then reactive perikarya and puncta increase in layers II-IVA and deep IVB and C, but are reduced in infragranular layers from 34W to term. From 30W positive somata form clusters in the cell-rich bands in layers IV and V and labelled neuropil in layers III and IV has a periodic pattern from 34W. Also from 34W, numerous lightly reactive pyramidal cells are present in layers II to IVA in primary, but not secondary, visual cortex. Our results show precocious expression of CB before full laminar differentiation of the cortex and that some of this expression is transient.