ABSTRACT: Macromolecules extracted from hydatidiform mole trophoblast inhibit mitogen-induced lymphocyte proliferation. To characterize the mechanism of this immunomodulation, we determined the effects of hydatidiform mole vesicle fluid (HMF) and tissue extracts (HME) on lymphokine function in vitro. Utilization of interleukin-1 (IL-1) and interleukin-2 (IL-2) were determined by using a lymphoma cell line (LBRM-33-1A5) and a murine T cell line (CTLL2), respectively. HMF suppressed (P < .05) IL-2-dependent CTLL2 cell proliferation at 500 (36.4% of controls) and 50 (74.9% of controls) μg/ml. HME also suppressed CTLL2 proliferation (P < .05) at 500 (46.0% of controls), 100 (67.2% of controls), 50 (71.5% of controls), and 10 (85.4% of controls) μg/culture ml. In contrast, HMF exhibited no effect on IL-1-stimulated LBRM-33-1A5 production of IL-2. However, 500 μg/ml of HME inhibited (P < .05) IL-2 production (63.0% of controls) in the IL-1 utilization assay. This suppressive effect was probably due to a carry over of HME from the LBRM-33-1A5 culture to the target cells (CTLL2) used to measure IL-2 production. Molecular weight chromatography of an HME sample eluted an IL-2 inhibitor in a low molecular weight (35–50 kd) and high molecular weight (> 250 kd) fraction. These data suggest that one way in which macromolecules derived from hydatidiform mole could interfere with in vitro immunologic responses is by modulating interleukin-2 function.