C57BL/6 mice (B6) which are homozygous at thenu (nude, athymic) andIpr(lymphoproliferation) locus (B6nulpr)are short-lived. We showed previously that increased survival could be obtained by grafting lymphoid cells from euthymic /pr-homozygous B6 mice (B6Ipr)mice ([Ipr→nulpr] chimaeras), but curiously enough not from normal (B6wild)mice ([wild→nulpr] chimaeras). Moreover female, but not male, [Ipr→tnulpr] chimaeras developed spleen and lymph node enlargement. In the present paper the distribution and absolute concentrations of all serum immunoglobulin (Ig) isotypes have been determined in these chimaeras and their controls. All chimaeras displayed whole serum Ig levels higher than those of B6wildmice, suggesting a successful reconstitution of the athymic recipients by the grafted lymphoid cells, but two types of chimaeras were peculiar. The short-lived [wild→nulpr] chimaeras showed a proportion of IgM as high as ungrafted B6nulprmice, suggesting a deficient down-regulation of IgM production by the grafted B6wild-typelymphoid cells. The [Ipr→tnulpr] female chimaeras recovered a long lasting overexpression of all Ig isotypes, like B6Iprmice, while all the other chimaeras showed a transient overexpression only. Since neither lymphadeno-pathy nor persistent increase of serum Ig levels were observed in [Ipr→nu] chimaeras, our data confirmed the need for a geneticallyIprhost to allow the significant development of theIprsyndrome.