The 1.5 to 3 µm gold particles used in the experiments described above, were large relative to cell size (ca. 10 µm). Since particle size usually should not exceed one tenth of the cell size , we also tried smaller particles, i.e. 0.7 µm tungsten particles. This, however, did not increase transformation efficiency. Tungsten may have toxic side effects, however we did not yet test smaller gold particles.