Abstract: The invention relates to the use as an immunostimulant of a composition comprising biomass derived from a methanotrophic bacterium-containing culture. The immunostimulatory effect of this biomass material renders this particularly suitable for use in preventing disease or conditions in an animal body which are associated with, caused by, or otherwise contributed to by any exogenous foreign material, e.g. a pathogenic microorganism. A preferred biomass for use in the invention is that derived from a microbial culture comprising Methylococcus capsulatus (Bath) (strain NCIMB11132), DB3 (strain NCIMB 13287) and DB5 (strain NCIMB13289), optionally in combination with DB4 (strain NCIMB13288).