Cyclin-cyclin dependent kinase (CDK) complex is negatively regulated by interaction with CDK inhibitors (CKIs). p27 protein is a major CKI in mammals and its down-regulation correlates with malignant transformation. However, some cancer cells express p27 at normal level, suggesting not only quantitative but qualitative control of p27, although little is known about such control. We analyzed the interaction between p27 and cyclin A (CycA)-CDK complex in living human cell lines, using a split yellow fluorescent protein (YFP) system in which the YFP fluorescence solely depends on p27-CycA binding. Introduction of this system into various cancer cell lines revealed that certain cell lines show no detectable YFP fluorescence. Furthermore, these cell lines exhibited reduced p27-CycA interaction as evaluated by immunoprecipitation, while they showed normal co-localization of both proteins. These results suggest that some cancer cells are defective for efficient interaction between p27 and CycA-CDK complex due to qualitative alteration(s).