C57BL/6 mice (B6) which are homozygous at the nu (nude, athymic) and lpr (lymphoproliferation) locus (B6 nulpr) are short-lived. We showed previously that increased survival could be obtained by grafting lymphoid cells from euthymic lpr-homozygous B6 mice (B6 lpr) mice ([lpr----nulpr] chimaeras), but curiously enough not from normal (B6 wild) mice ([wild----nulpr] chimaeras). Moreover female, but not male, [lpr----nulpr] 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 B6 wild mice, 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 B6 nulpr mice, suggesting a deficient down-regulation of IgM production by the grafted B6 wild-type lymphoid cells. The [lpr----nulpr] female chimaeras recovered a long lasting overexpression of all Ig isotypes, like B6 lpr mice, while all the other chimaeras showed a transient overexpression only. Since neither lymphadenopathy nor persistent increase of serum Ig levels were observed in [lpr----nu] chimaeras, our data confirmed the need for a genetically lpr host to allow the significant development of the lpr syndrome.