This transformation was studied at various concentrations of PHA and during different periods of incubation. The coelomocytes were separated into adhering and non-adhering cells. The adhering cells were separated into trypsin-sensitive and trypsin-resistant. Results have clearly shown (P less than or equal to 0.01) that the coelomocytes do transform to PHA. Only the trypsin-resistant adhering cells (5-10%) having a high phagocytic activity, were capable of inducing the transformation of the non-adhering coelomocytes. None of these sub-populations of coelomocytes did transform alone. These results reinforce the existence in earthworms of a T-cell like function and perhaps receptors similar to those observed in higher vertebrates and confirm the cooperation of two cell types to achieve this activity.