The accumulation of tubular aggregates in type II skeletal muscle fibres and fibrillo-granular inclusions in hippocampal protoplasmic astrocytes are characteristic lesions of apolipoprotein E deficient mice. Moreover these inclusions reacted immunocytochemically with an antibody specific to fragment 17-24 of the published sequence of Alzheimer's amyloid peptide. In an effort to evaluate the role of apolipoprotein E in the formation of these abnormal structures, we examined the tibialis anterior muscle and the hippocampus of several groups of animals including: (i) apolipoprotein E "knockout" mice which had been whole body irradiated with 1200 rads and bone marrow replenished with apolipoprotein E sufficient marrow; and (ii) three transgenic murine strains that had been genetically engineered to express either human apolipoprotein E2, E3 or E4 protein on an apoE deficient background. The results of this study showed that the presence of murine apolipoprotein E (even in subnormal levels in the serum) in irradiated bone marrow replenished mice and in all three (E2, E3 or E4) human apoE transgenic strains was sufficient to prevent the aggregation of sarcoplasmic tubules in the tibialis anterior type II muscle fibres. Similarly apolipoprotein E "knockout" bone marrow replenished mice and all three transgenic strains expressing the different human apolipoprotein E alleles reduced the number of the astrocytic inclusions in the hippocampus to levels not significantly different to those observed in control C57Bl6J animals. The data obtained in this study indicate that neurological and neuromuscular abnormalities found in apoE deficient mice are reversed when apoE protein is replaced in the circulation, either by bone marrow transplantation of normal apoE sufficient marrow, or by gene therapy with the apoE gene, albeit of human origin and irrespective of the allele used.