Plasma samples from 51 CFS cases and 53 healthy controls were diluted 1:50 and examined for seroreactivity to bands corresponding to Gag (p30 or p68/80) and/or Env (gp69/71 or p15E) proteins present in only the infected lysate and not the uninfected lysate. This evidence concerns the gene ERVW-1 and myalgic encephalomeyelitis/chronic fatigue syndrome.