A one-dimensional model with size-resolved cloud microphysics is used to investigate possible reinforcement or inhibition effects of different concentrations of cloud condensation nuclei (CCN) on precipitation formation in lowlevel, warm stratiform marine and continental clouds. Precipitation is found to be inhibited due to newly activated droplets when a precipitating marine cloud moves over a polluted environment with higher CCN concentrations. Precipitation can be initiated in a non-precipitating cloud having a high concentration of small droplets by seeding giant CCN (> 1 mu m in radius) into the cloud. Precipitation intensity in warm, stratiform clouds with and without background giant CCN differs little, implying very limited effects of background giant CCN on precipitation formation. The limited effects include a slightly slower precipitation formation, which is found to be caused by lower supersaturation, in clouds containing the background giant CCN compared to clouds without giant CCN. The delay of precipitation development is longer for continental clouds with high CCN concentration than for marine clouds with low CCN concentration. Sensitivity tests show that by increasing the giant CCN concentration an earlier precipitation can be initiated, especially in higher CCN concentration cases. It is concluded that the role of giant CCN on precipitation formation in warm, stratiform clouds is minor and is only important when concentrations of both the CCN and giant CCN (especially the largest CCN, e.g., > 5 mu m) are high. (c) 2005 Elsevier Ltd. All rights reserved.