Calreticulin (CR) is widely recognized as a new human autoantigen but there are conflicting data concerning its relationship with the Ro(SS-A) ribonucleoprotein (RNP). Recent evidence suggests that CR binds to 52 kDaRo (Ro52) by a protein/protein interaction and binds to hY RNA and rubella virus RNA. Other studies have shown that initiation of immunity to either Ro52 or 60 kDaRo (Ro60) can lead to reciprocal spreading of autoimmunity to Ro60 or Ro52, respectively, and induce anti-La autoantibodies in some strains of mice. These findings support a physical association of these polypeptides in Ro/La complexes. To test the hypothesis that CR is physically associated with Ro52 and/or Ro60 we examined the sera of Ro52-, Ro60- and La-immunized mice for intermolecular spreading to CR. Immune sera from BALB/c and C3H/HeJ mice immunized with recombinant 6xHis-mouse Ro52, 6xHis-human Ro60 or 6xHis-human La were tested for reactivity by ELISA and immunoblotting with a full-length human CR protein expressed as a soluble maltose binding protein fusion protein (CR-MBP). Five of the six Ro52-immunized C3H/HeJ mice sera and all six Ro60-immunized C3H/HeJ mice sera reacted with the CR-MBP (but not a MBP control) on ELISA. In the BALB/c group, the responder rate was lower with one in six of the Ro52-immunized and one in five of the Ro60-immunized mice spreading to CR. In contrast, none of the BALB/c or C3H/HeJ mice which was immunized with La showed evidence of a recruited anti-CR antibody response. Immunoblotting of the different recombinant proteins with immune sera from the C3H/HeJ mice confirmed the specificity of the initial and recruited antibody responses. The spreading of immunity from Ro52 and Ro60 to CR in Ro-immunized mice suggests that a subpopulation of CR or CR-like molecules must associate under certain circumstances with Ro52 and Ro60 polypeptides in vivo, possibly as Ro/CR complexes concentrated in surface membrane blebs of apoptotic cells. The lack of spreading to CR in La-immunized mice suggests that CR may be associated with a subpopulation of Ro particles from which La has already dissociated.