Not nocuous bacteria are important for the maturation and the modulating activity of the gut immune system. However, the humoral immune response against commensal and probiotic bacteria is less documented, particularly in farmhouse animals. Blood serum and saliva were collected in two trials where probiotics, Lactobacillus rhamnosus GG (LbR), well-defined human isolate (Trial A), and a novel and abundant porcine commensal, Lactobacillus sobrius strain 001T (LbS) (Trial B), were supplemented to weaning pigs. Anti-LbR IgA were present in serum of pigs before treatment with LbR, but also after 1 or 2 weeks in control pigs, notwithstanding the absence of DNA from LbR in colon. Pigs fed or not LbS for 1 or 2 weeks had LbS-specific IgA, in saliva and in serum. Colon contents of control subjects were positive for DNA from LbS. We hypothesized that part of this IgA strain-specific activity is partially related to immune cross-reactivity between different Lactobacillus-species. With the procedure of Shu et al. [Shu, Q., Bird, S.H., Gill, H.S., Rowe, J.B., 1999. Immunological cross-reactivity between the vaccine and other isolates of Streptococcus bovis and Lactobacillus. FEMS Immunol. Med. Microbiol. 26, 153¿158], after ELISA test on blood serum or saliva pre-incubated with LbR or LbS, each strain blocked a relevant part of IgA specific for the other. So bacteria with different affinity for the pig present reciprocal crossed immune activity. When probiotics are supplied to weaning pigs, the possible action of already present multi-effective IgA should be considered. The mechanism of IgA induction by certain probiotics needs to be addressed in further studies.