Adult, healthy Manila clams, Ruditapes philippinarum, were inoculated in the laboratory with different, possibly pathogenic bacteria species to study the cellular immune responses. Challenging the clams with Vibrio P1, the causative agent of brown ring disease, induced significant increased total hemocyte counts three days post-challenge. The triggering role of the number of bacteria in the inoculum suggested a threshold for inducing the host response. In addition, a significant decrease of hyalinocytes and an increase of granulocytes revealed a modification in hemocyte population balance. No response was observed in individuals that received either heat killed-Vibrio P1, V. anguillarum or V. pelagius. These experiments demonstrate in R. philippinarum the influence of bacterial parameters on the selective induction of host immunodefence mechanisms. The biological processes by which intra-pallial inoculation of Vibrio P1 stimulates the internal defence system of the host has not yet been identified but the role of some bacterial factors (toxins, adherence capacity) is discussed.