To the Editor: In a recent prophylactic trial, Balfour et al. (May 25 issue)1reported a significantly reduced incidence of cytomegalovirus excretion in blood and urine in renal-transplant recipients given high doses of acyclovir from the time of transplantation. Active cytomegalovirus infection was defined by the isolation of cytomegalovirus from cultured specimens from any site. The exact details of the procedure used for viral isolation were not given, but it can be assumed that this involved the appearance of cytopathic effect in inoculated cell cultures. Therein lies a problem, since the very specimens that are to be tested for the.Â .Â .