Abstract: AIM To study the influence of natural antikeratin autoantibod ies (AK autoAbs) on cell cycle of human keratinocytes cultured in vitro. METHODS Natural antikeratin autoantibodies were purified from pooled human serum with affinity chromatograph y, and purity analysis conducted with sodium dodecyl sulphate-polyacrylamide gel electrophore sis (SDS-PAGE). Influence of purified AK autoAbs on cultured human keratinocytes was stud ied with MTT colorimetric determination and cell cycle analysis. RESULTS Purified antibodies were pure enough for electrophoretic analysis. Results from MTT colorimetric determin ation demonstrated that AK autoAbs have significant dose-dependent inhibitory effect on cultured keratinocytes. Correlation coefficient was -0.797. Cell cycle analysis showed th at after treatment with 0.01125 g*L-1 AK autoAbs for 72 h, the proportion of cells in G2/M + S phase decreased from 56.2% to 46.7%, and the proportion in G1/G0 phase correspondingly increased 43.8% to 53.3%. CONCLUSION Natural AK autoAbs, purified fro m normal human serum, have inhibitor y effect on the proliferation of human keratinocytes cultured in vitro.