MRC-5 (Medical Research Council cell strain 5) is a widely used human embryonic lung fibroblast cell line, established in 1966 from the normal lung tissue of a 14-week-old male foetus. It is a diploid cell strain used for vaccine production (hepatitis A, rabies, varicella), virology research, and cytotoxicity testing. MRC-5 cells are certified by regulatory authorities as safe for human vaccine manufacture.