Mutants resistant to 16 individual antibiotics were isolated from 2 fast-growing and 2 slow-growing strains of Lotus rhizobia and their symbiotic effectiveness on L. pedunculatus evaluated. Resistance to streptomycin, spectinomycin, chloramphenicol and tetracycline (inhibitors of protein synthesis) was associated with little or no loss of effectiveness with all 4 strains but resistance to nalidixic acid and rifampicin (inhibitors of nucleic acid synthesis), and to D-cycloserine, novobiocin and penicillin (inhibitors of cell wall-cell membrane synthesis) was associated with significant loss of effectiveness in 20-100% of the mutants. Resistance to viomycin, neomycin, kanamycin, and vibramycin was associated with loss of effectiveness with mutants of the 2 fast-growing strains but not with mutants of the 2 slow-growing strains. When tested on 4 alternate host legumes individual mutants of a slow-growing strain showed significantly different levels of effectiveness. Both the inherent characteristics of the bacterium and of the host plant will probably influence the symbiotic effectiveness of antibiotic-resistant mutants of Rhizobium.