Latent VH immunoglobulin allotypes are expressed unexpectedly and transiently at low concn in the serum of rabbits. Latent group a1 molecules in sera from rabbit colonies in Philadelphia (U.S.A.) and Birmingham (U.K.) were examined for a1 specificity and clonal diversity using reference nominal allotypic reagents and isoelectric focusing (IEF) autoradiography. Latent a1 molecules from rabbits of both colonies had diverse spectrotypic patterns in the pI range 5.5-8.3, as identified with 125I-labelled affinity-purified specificity-tested, anti-nominal a1 antibody. Comparisons of spectrotypes between nominal a1 antigen and latent a1 focused molecules revealed a marked correspondence in banding over the pI range. Reference anti-nominal a1 antibodies could be absorbed out substantially by the IgG fraction of serum from two rabbits containing latent a1 molecules; in a reciprocal fashion absorption with nominal a1 molecules reduced the binding of focused latent a1. The latent a1 molecules from both U.S.- and U.K.-bred rabbits displayed strikingly similar IEF spectra and their antigenic similarities were confirmed by similar absorption capacities of the reference anti-a1 serum. When sequential serum samples from one (U.S.) latent a1 rabbit were compared by IEF, some bands, e.g. those between pH 7.75 and 8.3, appeared to fluctuate in their presence, whereas others, e.g. between pH 5.3 and 7.4, were expressed continuously. We can conclude that latent a1 molecules are clonally complex and some are consistently produced in small amounts. As they also show antigenic similarity, if not identity with nominal a1, we believe that they are probably the product of the same gene (or genes) with an equivalent capacity to be associated with specificity-determining genes even though the level of synthetic activity is lower and possibly governed differently. Anti-a1 antibody was raised in a rabbit in which latent a1 allotype had been previously detected. This antibody was of low avidity and, while inhibitable on RIA by nominal a1 it was not inhibitable by the donor's latent a1 or by a second latent a1 of the same (partially inbred) U.K. colony, but was inhibitable by a latent a1 serum from the Philadelphia, U.S.A. colony. This result suggests that a1 molecules are the products of more than one gene.