The yields of unstable chromosome aberrations induced in human lymphocytes by αparticles from plutonium-239 have been measured. Plutonium citrate solution was mixed with heparinized blood so that doses of 13-160 rad were delivered in 24 hours. Dicentric aberration yields (Y) fitted best to the linear expression Y = 3·72 ± 0·23 × 10-3 rad-1. Inclusion of a 6·5 rad point resulting from a 1·7 hour irradiation raised the yield coefficient insignificantly to 3·75 ± 0·24 × 10-3. The aberration yields are in good agreement with data from curium-242 αparticles obtained in this laboratory but they are much lower than those obtained in two other laboratories. Reasons for this disagreement are examined.