Measurements of R and a search for heavy-quark production in e+e- annihilation at sqrt s =50 and 52 GeV.

The R ratio for e/sup +/e/sup -/ annihilation into hadronic final states is measured to be 4.34 +- 0.45 +- 0.30 and 4.23 +- 0.20 +- 0.21 at c.m. energies of 50 and 52 GeV, respectively. From these values of R and an analysis of the event shapes we deduce a 95%-confidence-level upper limit for the production rate of new heavy charge +(2/3e or -(1/3e quarks to be 0.19 units of R.