After monoassociation of Bifidobacterium longum with germ-free BALB/c mice (nu/+, heterozygous to nu gene), B. longum was recovered (10(2)-10(4) viable units per organ) from the mesenteric lymph nodes, liver and kidneys for 2 weeks post intragastric inoculation, but not after 4 weeks and later. Cessation of bacterial translocation was not observed in monoassociated nude (nu/nu) mice. Anti-B. longum IgA antibody was detected by ELISA using B. longum cell wall antigen in bile from Week 1 and in ileac wall extract from Week 8 post-association in both nu/+ and nu/nu mice. Total IgA levels in bile, ileac wall extract and caecal contents were also elevated in both mice after monoassociation. Cell-mediated immunity measured by the footpad test and macrophage migration inhibition test using B. longum protein fraction was detected in nu/+ mice in Week 4 and later, but not in nu/nu mice. Nu/nu mice reconstituted 4 weeks prior to monoassociation with lymphocytes from flora bearing nu/+ mice developed delayed footpad reactivity and bacterial translocation stopped after 4 weeks. Cell-mediated immunity rather than IgA antibody correlated well with the cessation of translocation.