The effect of dry residues from several plants, belonging to different families on certain microorganisms in vitro and in vivo, was studied. Dry residues of paprica leaves, tomato tops, egg plant leaves, guava leaves, onion peels, garlic tops, wheat straw, sugar cane leaves, cotton leaves, Egyptian clover tops, field bean tops or pea tops were examined for the presence of antibacterial substances, using successive extractions with hexane, ethyl ether, ethanol, and water, respectively, for each plant residue. On culture media, the antibacterial effect, expressed as width of inhibition zones, differed according to the type of plant, type of micro-organism, and extraction medium, used for each plant. Water extract from each of the studied plants showed no effect on any of the studied micro-organisms, while the other extracts indicated the presence of antibacterial substances in all the used plants. In most cases, ether extract showed the highest incidence of antimicrobial activities against the majority of test micro-organisms. In general, the antibacterial substances seemed to be more inhibitory to Gram-positive bacteria than to Gram-negative ones. Ethyl-ether extract of the residues of most of these plants markedly affected the growth of more than one of the different Rhizobium species when grown on culture medium, as indicated by the presence of wide zones of inhibition.