Instruments on NASA's DC-8 Airborne Laboratory were used to obtain measurements of heterogeneous ice nucleating aerosols (IN) and CN (condensation nuclei, particles >0.012 &mgr;m diameter) in background and aircraft-affected air during the April--May 1996 SUCCESS project (Subsonic Aircraft: Contrail and Cloud Effects Special Study). IN were measured with a continuous flow diffusion chamber over a range of processing conditions from about -15 to -40 ¿C and from ice saturation to ~15% water supersaturation. IN concentrations ranged from <0.1 to ~500 l1, being generally greater at colder temperatures and higher supersaturations. Within limited sample periods, IN concentration related strongly to sampling temperature and supersaturation, but overall, there was wide variability. IN and CN concentrations did not generally correlate. During penetrations of aircraft exhaust plumes, CN exhibited a very strong response, but there was no strong evidence that exhaust is a significant IN source for the temperature and supersaturation conditions of our measurements. ¿ 1998 American Geophysical Union