Measurements of condensation nuclei (CN) and cloud condensation nuclei (CCN) were obtained from aircraft penetrations of controlled burns of chaparral, pine forest, and a JP-4 (jet fuel) pool fire. Vegetative burns gave rise to large numbers of CN, most of which were also CCN at a supersaturation of less than 1%. This is to be contrasted with the much lower activity of smoke from JP-4 burns which gave only 1--2% CCN activity under identical conditions. The field observations are consistent with laboratory results under conditions which simulate natural clouds. This implies that although droplets readily grow on smoke from vegetative burns leading to activation of a significant number of the particles, mutual competition ensures that a large number of interstitial haze particles remain. By contrast, in clouds produced from smoke from jet fuel combustion, a larger fraction of the nonactive interstitial particles remain after cloud formation. Penetrations of a forest fire capping cloud show high concentrations (>104 cm-3) of small (2 μm diameter) but optically active particles together with high concentrations (5¿105 cm-3) of nonactivated haze particles. Photogrammetric measurements of vertical velocities of clouds from controlled burns showed that weakly sheared plumes penetrated upward more effectively than strongly sheared plumes with cloud and smoke cap velocities as large as 20 m s-1. This implies vertical velocities twice this value and cloud supersaturations of ~2% under the observation conditions. Implications for particle removal by in-cloud scavenging and precipitation are discussed. ¿ American Geophysical Union 1991