Various studies have shown that complement components are synthesized by renal cells and that mRNA for a number of complement components is detectable in renal tissue. The present study shows that complement proteins are present both in fetal and adult human kidneys. The localization of the complement components was compared with the localization of other proteins for which specific expression in defined renal cell types is known from the literature. In adult human kidneys C3, factor B and factor H were detected in the mesangial area by immunohistochemistry, whereas C2 and C4 were present in the proximal tubuli. In fetal kidneys C3 and factor B were expressed in glomeruli of kidneys of 11 weeks of gestation. In kidneys of 13 to 19 weeks of gestation no staining for C3 was found in the glomerulus, whereas for factor B glomerular staining was found in all fetal kidneys examined. Factor B was also detected in fetal tubuli and in the interstitium. Factor H was expressed in fetal tubuli starting at 13 weeks of gestation. For both C3 and C2 weak tubular staining was found in all fetal kidneys investigated. C4 could not be detected in any of the fetal kidneys. While not all the complement proteins investigated were detectable by immunohistochemistry, by RT-PCR analysis, mRNA expression for C3, factor B, factor H, C2 and C4 was found in all adult and fetal renal tissue. The finding of mRNA for the complement components in the fetal and the adult kidneys indicates that local synthesis of complement occurs both in the adult and in the fetal kidney. Next to the in situ expression of complement components in fetal kidneys the synthesis of complement proteins in vitro by fetal renal cells was investigated. Four different primary mesangial cell lines were shown to synthesize all complement proteins investigated. Although a specific role for complement during the development of the kidney is not known, it is possible that certain complement components may play a role during renal differentiation.