Schiff bases and hydrazones of substituted isatins (1-28) were prepared by reacting isatin and aromatic primary amines/hydrazines. A new series of the corresponding N-Mannich base (29-35) was synthesised by reacting them with formaldehyde and diphenyl amine. The chemical structures were confirmed by means of 1H-NMR, IR spectral data and elemental analysis. The compounds were screened for antibacterial activity against seven Gram (+) and seven Gram (-) standard and pathological bacterial strains by the paper disc diffusion technique. The minimum inhibitory concentrations of the active compounds were determined. 1-Diphenyl amino-methyl-3-(4-bromo phenylimino)-1,3-dihydro-indol-3-one (30) and 3-(4-bromo phenylimino)-5-nitro-1,3-dihydro-indol-3-one (13) were found to be the most active compounds of the series. Mannich bases exhibited higher activity than the corresponding Schiff bases.