Biological Nitrogen Fixation (BNF) is one of the major sources of nitrogen to inherently low tropical soils (Bohool et al.,1992). The level of resistances of microorganisms to different concentrations of antibiotics is a factor for determining the degree of tolerance of microbes to stressful environments, especially microbes which take part in BNF. Carriers are also required for effective inoculant preparation to facilitate the survival and multiplication of rhizobium strains(microbes). Parent rhizobium strains USDA 3384 and USDA 3451 were exposed to antibiotics at different concentrations to determine their intrinsic resistance level. These were further seeded into three different solid carriers (peat, composted maize cob and cowdung). The developed mutant strains inoculated with the solid carriers and mutant strains inoculated without the solid carriers were evaluated in the laboratory and pot experiments and their effect with or without solid carriers on growth and dry matter yield of cowpea (Ife bimpe) were assayed. Data on plant growth and dry matter yield of cowpea were analysed using ANOVA and means were separated using Duncan Multiple Range Test (DMRT) at 5 % probability level.
USDA 3384 and USDA 3451 were resistant to Ampicillin trihydrate pensy at 500 mg/l and Streptomycin sulphate at 1000 mg/l respectively
