The addition of 10 nM staurosporine (ST) to MDA 361 breast carcinoma cells induces a G1 arrest, which correlates with the loss of the catalytic activity of the G1-associated cyclin-dependent kinases (cdks) and increased levels of underphosphorylated retinoblastoma protein. This treatment resulted in a slight but detectable reduction in the protein levels of cdk6 but did not reduce the levels of cdk2, cdk4, or the D cyclins. The level of cyclin E declined initially but returned to normal levels 24 h after exposure to 10 nM ST. Because the levels of the G1 cdks and cyclins did not correlate with loss of kinase activity, the role of the cdk inhibitors involved in regulating the activity of the G1-associated cdks was investigated. The significant reduction in cdk activity observed in MDA 361 cells treated with ST for 24 h correlated with increased levels of p18 and p27Kip. The inhibition of kinase activity of preformed cdk2 complexes by lysates of MDA 361 cells that had been treated with 10 nM ST for 24 h was shown to be due to p27Kip. The reduction in the level of the active phosphorylated form of cdk2 also correlated with an increase in the level of p27Kip, which has been shown to inhibit the phosphorylation of the activating Thr-160 residue of cdk2. These results indicate that treatment of MDA 361 cells with 10 nM ST induces a significant increase in the levels of several cdk inhibitors that appear to be responsible for the observed G1 arrest.