In situ measurements of jet engine exhaust from a Sabreliner were made by instruments on board the NCAR Electra during a brief period of coordinated flying. Particle size distribution and critical supersaturation spectra were monitored before, during, and after the encounter with the jet exhaust plume by a condensation nucleus counter, an active scattering aerosol spectrometer probe (ASASP), and a cloud condensation nuclei (CCN) spectrometer. The relationships between particle size and corresponding critical supersaturation in the background air and within the jet exhaust plume are developed. Results indicate that background particles are much more active as CCN than exhaust particles of the same size, of which less than 1:100 are active at 0.8% supersaturation. An estimate of the engine sooting efficiency lies between 3¿10-5 and 10-6. ¿ American Geophysical Union 1991