The nonsteroidal anti-inflammatory drugs have been shown to support cytoprotection of cells by shifting cells toward a quiescent state (G 0 /G 1 ). Extracellular signal-regulated kinase (ERK) is required for cells to pass from G 1 phase into S phase, and macrolide antibiotics can inhibit ERK1/2 phosphorylation. However, previous reports suggest that macrolide antibiotics do not affect cell growth in bronchial epithelial cells. Therefore, we studied normal human bronchial epithelial (NHBE) cells to determine whether clarithromycin (CAM) suppresses ERK, delays bronchial epithelial cells from progressing to S phase, and delays cell growth. Exposure to CAM at 10 μg/ml daily over 4 days irreversibly decreased the cell proliferation with and without growth supplements ( P < 0.0001). CAM also inhibited ERK1/2 phosphorylation over the first 90 min of exposure ( P < 0.05 for 30 min, P < 0.0001 for 60 min, and P < 0.01 for 90 min) and decreased the ratio of phosphorylated ERK1/2 (pERK1/2) to total ERK1/2 (tERK1/2) ( P < 0.0001). Incubation with CAM for 48 h increased the proportion of cells in G 1 phase (means ± standard deviations) from 63.5% ± 0.9% to 79.1% ± 1.4% ( P < 0.0001), decreased that in S phase from 19.8% ± 1.2% to 10.0% ± 2.1% ( P < 0.01), and decreased that in G 2 /M phase from 16.7% ± 0.4% to 11.0% ± 0.8% ( P < 0.001). In contrast, the ratio of pMEK1/2 to tMEK1/2 was not altered after exposure to CAM. These results suggest that macrolide antibiotics can delay the progression of NHBE cells from G 1 phase to S phase and can slow cell growth, probably through the suppression of ERK1/2.