Small wire implants of titanium or stainless steel were inserted into mouse leg muscles to test the reaction of regenerating skeletal muscle. Muscle fibres regenerated rapidly, starting at 3 d; by 2 wk all implants were encapsulated in thin (10-mu-m) fibrous tissue capsule surrounded by myotubes for 15-20-mu-m. Quantitatively there were no detectable differences in muscle regeneration between the two metals. The initiation of myoblast precursor cell replication was determined in regenerating muscle next to the implants. Tritiated thymidine was injected 18-156 h after implant insertion and labelled myotube nuclei in the regenerated muscle indicated that their precursors had started DNA synthesis 24 h after implant insertion. This is similar to myogenesis in many other muscle lesions and indicated that neither titanium nor stainless steel retarded muscle regeneration.