Summary It is widely acknowledged that a mother, and the maternal environment, can have a profound effect on the phenotype of her offspring [1]. For example, immunocompetent vertebrate mothers can pass on immune factors to their otherwise immune naive offspring [2]. Recently, it has also been demonstrated in invertebrates that maternal contact with pathogens and immune elicitors can have a positive impact on offspring resistance [3] and immune system components [4] and [5]. Here, using a cross-fostering design, we show that trans-generational immune priming in a social insect, similar to birds [2], takes place through factors in the eggs, with effects persisting into adult worker offspring. Additionally, eggs from immune-challenged mothers are imbued with increased internal antibacterial activity when they are laid.