The mechanism for tissue-specific differences in apolipoprotein B (apoB) mRNA editing efficiency is not known. Structural data are presented which demonstrate tissue-specific, quantitative differences in the high order complexes containing apoB mRNA editing activity and RNA-binding proteins. The bulk of rat enterocyte extract editing activity sedimented at 11 S with an additional 5-10% at 60 S. Rat liver extract activity was less abundant and only sedimented at 60 S. Ultraviolet light cross-linking revealed two protein activities of approximately 66 and 44 kDa which specifically associated with apoB RNA substrates and cosedimented with editing activity. Extracts differed in the cross-linking yield of p66 and p44 and kinetically, enterocyte RNA-protein complexes reached maximum abundance more rapidly than those in liver extracts. Both 60 and 11 S forms of the editing activity redistributed to 27 S during in vitro editosome assembly. The redistribution of editing activities was accompanied by a corresponding redistribution of p66/p44 to 27 S. The data demonstrate that p66 and p44 are common to liver and enterocyte 27 S editosome assembly processes and suggest that differences in both the pre-editosomal assembly state of editing factors and their abundance may be mechanistically important for tissue-specific differences in editing efficiency.