activation of b cells by 1 microm particulate lysozyme or peptides a thdependent pathway requiring cd40cd40 ligand interaction many antigens encountered by the immune system are included in complex structures such as bacteria or parasites we previously developed an in vivo model to study the immunogenicity of particulate antigens based on covalent linkage of proteins or peptides to 1 microm latex particles and showed that these antigens cannot be presented to mhc class iirestricted specific t cells by b cells however they induce strong cd4 t cell responses when injected to mice without adjuvant the present study demonstrates that four out of the five proteins tested did not stimulate antibody synthesis when linked to 1 microm microparticles although a strong igg production was induced by the same proteins administered in soluble form with adjuvant in contrast lysozyme and two synthetic peptides containing b and t cell viral epitopes induced strong and longlasting specific antibody responses when linked to 1 micrometer synthetic beads the isotypic pattern of antibodies induced by particulate lysozyme was similar to that induced by the soluble protein in alum studies using cd4 t celldepleted mice revealed that the induction of antibodies by particulate lysozyme strictly required th cell activity moreover the tb cell cooperation involved in b cell activation by antigens linked to beads required cd40cd40 ligand interaction thus these particulate antigens provide a useful tool to study the mechanisms of induction of antibody response against complex bacterial or parasitic antigens moreover they may represent attractive candidates to elaborate efficient new vaccines using short synthetic peptides