Human fibroblasts are the only cells that regularly produce cytomegalovirus of adapted laboratory strains and release fair amounts of cell free virus. One such strain, Towne, was adapted to WI-38 and could release 10(6) to 10(7) p.f.u. per ml tissue culture fluid. The maximum cell-free virus is released at 5-7 days after infection, when cytopathic effect has reached a peak. The virus harvest must be combined with stabilizer rapidly and rapidly lyophilized or frozen, for the virus is unstable even at 4 degrees C. With regard to control, cytomegalovirus is poorly neutralized by antibody-containing serum, and complement is required for maxium neutralization. Frequent addition of fresh antiserum may be necessary to keep cytomegalovirus from replicating in cells inoculated to detect other viruses. Another special problem is the putative association of oncogenicity with herpes group viruses. Tests must be done in newborn hamsters to show that live or inactivated virus does not cause tumors. Towne vaccine tested in man has produced asymptomatic infection with antibody response. No urine excretion has been observed, and antibodies persist for at least 2 years.