Three antagonists, i.e. Pseudomonas fluorescens, Bacillus subtilis and Serratia spp., were tested either alone or as a mixture for suppression of onion neck rot and black mould diseases under field conditions. Samples were taken for determination of total phenols, cartenoides and flavonoids. The average of disease reduction was 18.75 % for mixture compared to 47.0% in the non-treated control. In addition to disease suppression, treatment with a mixture of the antagonists promoted plant growth in terms of increased bulb yield. The mixture of the tested bioagents was more effective in reducing the disease and promoting both plant growth and bulb yield compared with using each of them alone. The result indicated the biocontrol efficacy and storage stabilities were not increased with the enhancement of the concentration of biocontrol agents. Treatment the onion bulb with the bioagents increased cartenoides in mixture treatment and the fungicide Folucolon compared with the control and another tested treatment. Treatment onion bulb with any of the tested bioagents or their mixture decreased total phenols compared with the fungicide Folucolon and control in the two concentrations (108 CFU/ml and109 CFU/ml). On the other hand, they also increased flavonoids compared with control and the fungicide Folucolon in the two concentrations.