Southern analysis has previously shown that the rabbit genome contains multiple genes coding for the constant regions of IgA heavy chains. In the present study, clones containing these C alpha genes have been isolated from cosmid and phage libraries. Restriction mapping and Southern analysis of the clones identified 13 non‐allelic C alpha genes; 11 of the genes were clustered in individual or overlapping clones. The clustered genes are separated by 8‐18 kb, and in total, the C alpha genes span a minimum of 160 kb of DNA. Southern analysis has shown that all genes within a cluster have the same transcriptional orientation, and that switch sequences are present 5′ of at least 12 of the 13 genes. The nucleotide sequence of each C alpha gene was determined, and it appears that all genes are functional; thus, rabbit may have as many as 13 IgA isotypes. Comparisons of the protein sequences encoded by the 13 C alpha genes showed that the CH2 and CH3 domains of the alpha‐chains are highly conserved, whereas the CH1 and hinge regions are highly diverse. Southern analysis of genomic DNA samples from other species within the order Lagomorpha showed that all samples had multiple C alpha hybridizing fragments. Thus, it is likely that all lagomorphs have multiple IgA isotypes and hence complex secretory immune systems.

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Rabbit secretory immunoglobulin A (sIgA) was digested with papain in the absence of reducing agent and four fractions were isolated by gel filtration. Three of the four fractions, undigested sIgA, Fc~,, and Fabz, have previously been characterized (Hanly et ai. (1973), Biochemistry 12, 733); the fourth fraction (approximately 15% of the total protein) was identified as Fab,-like material by its sedimentation and antigenic properties. Each of the four fractions was analyzed for the allotypic specificities controlled by the f and g loci by quantitative precipitation of the radiolabeled fractions with anti-f or anti-g antisera. The f allotypic specificities

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