Human cytomegalovirus (CMV) replication begins with the expression of two regulatory proteins, IE1-491aa and IE2-579aa, produced from differentially spliced transcripts under control of the ie1/ie2 promoter-enhancer. A deletion mutation removing all 406 IE1-491aa-specific amino acids was engineered into the viral genome and this mutant (RC303-DELTA-Acc) was propagated on an IE1-491aa-expressing human fibroblast cell line (ihfie1.3). RC303-DELTA-Acc failed to replicate on normal human fibroblasts at low multiplicities of infection (mois). At mois gt 3 plaque-forming units per cell, virus replication and production of progeny were comparable to wild type. However, at mois between 0.01 and 1, mutant virus replicated slowly on normal fibroblasts, a pattern that suggested initiation of productive infection required multiple hits. Replication of RC303-DELTA-Acc correlated with the ability to express IE2-579aa, consistent with a role for IE1-491aa in positive autoregulation of the ie1/ie2 promoter-enhancer and with data suggesting that virion transactivators compensate for the lack of IE1-491aa under high moi conditions. ie1-deficient CMV should be completely avirulent, suggesting its utility as a gene therapy vector for hematopoietic progenitors that are normal sites of CMV latency.