Eight mutant hybridoma lines are described, which synthesize short immunoglobulin mu chains. Four internal deletions were mapped by Southern blot analysis. They are shown to remove DNA from either part or all of the first, and first and second, constant mu exons. The sizes of the deletions range between 0.6 and 5 kb, leaving an equal or unequal number of splice signals. Shorter mu RNA of one size was found irrespective of whether an exon was completely or only partially deleted. These results preclude exclusive 3' (constant region) to 5' (variable region) directional splicing of the mu RNA. No important signals seem to reside in the deleted DNA stretches affecting the transcription or the correct RNA splicing of the remaining exons. The internal mu protein deletions revealed unusual covalent light chain attachment demonstrating functional homology between the first (normally used) and fourth mu constant domain. The other mu protein deletions (10, 11, and 12 kd) involved neither gross DNA nor RNA lesions and are considered to be due to premature chain termination. Since secretion is found in most of the mutant IgM-producing lines, no single one of the four mu constant domains (including the C-terminal one which contains the so-called secretory piece) is necessary for secretion.