To address the effect of an agglutogen on virus infection, we studied the avidin-associated inhibition of infection by biotinylated M13 phages (BIO-phages). Microscopic observation of mixtures of BIO-phages and avidin-fluorescein conjugates revealed many aggregates. Even at low phage concentrations, avidin induced inhibition of infection significantly. Anti-M13 phage antibody also made aggregates and inhibited the infection but in a different manner from avidin. The inhibition by avidin was at > or = 2 microg/ml, time dependent and marked until 10 min after the mixing of the BIO-phages and Escherichia coli. On the other hand, antibody inhibited the infection at > or = 0.1 microg/ml dose dependently, and the inhibition was time dependent and marked until 45 min after the mixing at moderate and low phage concentrations. These results indicate that avidin against BIO-phages and antibodies are agglutogens, and the inhibition of the BIO-phages by avidin is closely related to the tetramerization of avidin. Agglutogens may be novel alternative antiviral drugs.