Incubation of bovine peripheral blood monocytes with bovine viral diarrhea virus (either Singer or NY-1 strain) caused a consistent, statistically significant decrease in their random locomotion (no chemoattractant) and chemotaxis towards a chemotactic lymphokine. Chemotaxis was determined by a modification of the Boyden method. Incubation of bovine viral diarrhea virus with mononuclear cells depressed chemotaxis by a mean of 56% (P < 0.0005). Heat-killed virus had no effect on monocyte motility. Data suggest that bovine viral diarrhea virus can rapidly suppress monocyte functions in vitro, but by unknown mechanisms, not by killing cells.