Abstract
Four strains of Rhizobium trifolii (28B, 35B, 73B and 8CB), having different intrinsic resistances to antibiotic markers, showed significant increases in nodule dry weight, nitrogenase activity and green fodder of two varieties of clover (BL-1 and BL-10). Strain 28B produced highest yields in both clover varieties; these characteristics were all positively correlated with yield, especially nitrogenase activity. The competitive ability of different strains varied with different host cultivars. Intrinsic antibiotic resistance was not found associated with loss of effectivity.