3-Chloro-4-(dichloromethyl)-5-hydroxy-2(5 H)-furanone (MX), a by-product of wood pulp manufacture and a contaminant of chlorinated drinking water, was investigated for potential teratogenicity using the micromass in vitro test system. Twelve-day rat embryo midbrain (central nervous system, CNS) and limb bud (LB) cells were exposed to MX at concentrations of 1, 2, 5, or 10 mug/ml in the culture medium with or without S9 mix. Under the experimental conditions, the amount of MX rapidly declined in the culture medium with a half-life of 56 min. Nevertheless, differentiation of CNS and LB cells was significantly inhibited at concentrations of 2 mug/ ml or more, when the cells were exposed to MX in the absence of S9 mix. The estimated IC50 was approximately 3 mug/ml for both CNS and LB cell cultures. On the other hand, exposure of CNS and LB cells to MX along with S9 mix did not reduce the number of differentiated foci at any concentrations tested. These results suggest that MX may be a potential direct-acting in vitro teratogen.