[en] With the use of a new feedback levitation electrometer (with an increase in sensitivity by 10³ in comparison to our previous graphite experiments) iron objects of mass approx. 2 x 10^-4 g have been explored for fractionally charged quarks and/or a possible electron-proton charge difference. Upper limits found were N (quarks)/N (nucleons) < 3 x 10^-21 and (Q/sub p/ - vertical-bar Q/sub e/ vertical-bar)/Q/sub p/ < 10^-21. The present ''sensitivity'' is approx. 10⁷ times that of the original Millikan experiment