Normal human lung fibroblast (NHLF) proliferation is discoidin domain receptor (DDR)2, but not DDR1 dependent. NHLFs were reverse transfected with DDR2-, DDR1-specific small interfering (si)RNA or negative control siRNA using lipofectamine RNAiMAX. At 48 h after transfection, NHLFs where transferred to a 96-well black walled plate at 3,000 cells/well. Cells were serum-starved for 24 h and incubated with collagen I (25 μg/mL) or vehicle (acetic acid, 0.1 M) for an additional 16 h. Cell proliferation was determined using the DELFIA proliferation assay kit and measuring time-resolved fluorescence at 340 nm excitation and 615 nm emission. Results are representative of mean fold increase ± SD of two independent experiments performed in triplicate (n = 6, **P < 0.01).