In order to investigate inhibition of viral replication, human embryonic fibroblasts infected with cytomegalovirus (CMV) were treated with 0 to 25 micrograms/ml concanavalin A (Con A) and 0 to 150 micrograms/ml succinylated Con A (S-Con A). Alterations in cellular morphology occurred by day 2 post infection (p.i.) in cultures treated with 10 micrograms/ml Con A and 25 micrograms/ml S-Con A. With increasing concentrations of Con A and S-Con A there was decreased virus production from day 4 p.i. to day 10 p.i. Increasing levels of Con A and S-Con A also reduced the number of cells in culture. When virus titres were corrected to take cell number into account, decreased CMV titres in Con A and S-Con A treated cells appeared mainly to reflect decreased cell numbers. In support of this finding, in comparison with untreated CMV-infected fibroblasts, viral DNA synthesis was reduced and acid phosphatase levels were increased in CMV-infected cells treated with Con A or S-Con A.