The presence of a mononuclear cell reaction to 41 gestational choriocarcinomata, 10 invasive moles and 13 malignant trophoblastic teratomata has been investigated. The intensity of the reaction was graded; there was a significantly better response to therapy and survival rate in those with a "severe" cellular reaction than in those with a "mild" reaction to gestational choriocarcinoma. The pathological and clinical features of invasive moles showed no relationship with the cellular reaction to the tumour. The cellular reaction to trophoblastic teratomata was generally poor but there was a marked cellular reaction to the tumour of one patient who has enjoyed a sustained remission.The relationship of cellular reaction and response to treatment with other histological and clinical features was examined. With the exception of a positive correlation between the degree of vascular invasion and response to treatment, none was found.It is suggested that an infiltrate of mononuclear cells in gestational choriocarcinoma is probably a response to the presence of tumour antigens. The infiltrate favourably affects the response to chemotherapeutic agents, suggesting that it contributes to tumour cell death and it may be interpreted as an immunological response directed at tumour rejection.