Cytokeratin expression by the developing human enamel organ between the 10th and the 23rd gestational week was studied by indirect immunofluorescence microscopy technique using a panel of 15 monoclonal antibodies. The results showed that five antibodies (RKSE 60, Kk 8-60, EE 21-6, 6B10 and 1 C-7) were never reactive, that five antibodies (RCK 102, 188.8.131.52, Ks 19 and Pan 1-8) were always positive and that five antibodies (KB 37, RPN 11-62, Ks 13.1, Ks 8-12 and Ks 18.174) obtained or increased their positivity between weeks 12 and 13. It was concluded that a switch in cytokeratin expression occurred around the 12th-13th weeks. No further important change could be noticed after this period. So it is suggested that final cell differentiation was initiated at weeks 12-13.