During the course of mass spectral studies on the stability of bimetallic systems, an exceptionally abundant and stable Na6Pb cluster was discovered. For given exptl. conditions, an intense mol. beam consisting exclusively of the particle Na6Pb is generated via self-assembly from a mixed vapor of sodium and lead. While this observation is reminiscent of the discovery of C60 or Tl8C12, Na6Pb represents the first example of a binary finite-size 'alloy' (~0.5 nm diam.) exhibiting such extraordinary stability. [on SciFinder (R)]