In a previous publication we demonstrated that an anti-viral response against influenza can be achieved by immunization with a conjugate of the synthetic peptide corresponding to the sequence 91-108 of the hemagglutinin, when administered in complete Freund's adjuvant. In the present study we compare the adjuvant activity of the synthetic MDP with that of CFA and alum, in the above mentioned immunological system. The level of anti-peptide antibodies raised by the three adjuvants was similar, with only slight variations, yet, only CFA led to significant cross reaction with the virus. Nevertheless, MDP, when linked covalently to the conjugate (91-108)-TT was an efficient substitute for CFA in inducing anti viral protection against in vivo challenge infection. The administration of free MDP in a mixture with the peptide-toxoid conjugate did not lead to a significant protection.