Antagonistic interactions in mixed culture of methanotrophic bacteria Methylomonas methanica 12 and Methylocystis minimus 33 were investigated. The inhibitory action of Mcs. minimus exometabolites against Mm. methanica grown in liquid medium was found to be specific. Ultrafiltration established that the molecular weight of the substance having inhibitory activity lies within the range 2-10 kD. The activity is protease sensitive and relatively stable to heating. Electrophoretic analysis showed that a protein with molecular weight of approximately 8 kD prevailed in Mcs. minimus culture liquid. When Mm. methanica cells were incubated in culture liquid of Mcs. minimus, the sorption of the 8 kD protein by target cells was observed. This suggests that the inhibitory effect may be associated with the 8 kD protein which has properties similar to known bacteriocins.