A variety of mechanisms have been proposed to explain observed correlations between tropospheric climate variables and cosmic ray flux levels. The most direct hypothesis involves the formation of ice nuclei along ion tracks formed when cosmic rays pass through cloud water droplets. To test this hypothesis, we performed nucleation studies of a single water droplet undergoing repeated linear cooling cycles to -40 ¿C with and without a radioactive source in the vicinity of the drop. A comparison of the distributions of nucleation temperatures obtained with and without a source of 5.3 MeV α-particles present supports the null hypothesis that ionization tracks do not act as nucleation sites. In both cases we observe mean nucleation temperatures of -34.5 ¿C consistent with previous studies of homogeneous nucleation. ¿ 2001 American Geophysical Union