High fetal and neonatal brain levels of methyl mercury (MeHg) have been associated with the abnormal migration of neurons within the cerebellar and cerebral cortices. How MeHg interferes with cellular proliferation, migration, and differentiation is poorly understood. In this study, a cell recognition/cohesion assay based on the ability of dissociated neonatal mouse cerebellar cells to reaggregate was used to test whether MeHg exposure could disrupt cell surface recognition. Reaggregation of dissociated cells was monitored by measuring diameters from low-power photomicrographs. Exposure to 4 mg/kg body wt methylmercuric chloride (MeHgCl) 24 hr prior to the isolation of 3 days postnatal cerebellar cells altered the pattern of reaggregate growth. Between 25 and 51 hr in vitro (hiv), the exposed reaggregates grew at a faster rate than controls. Freshly isolated cells exposed in vitro to 0, 0.5, 1.0, and 4.0 microM MeHgCl initially exhibited a dose-related inhibition in reaggregate growth with an IC50 of 1.5 microM at 24 hiv. Following initial inhibition, exposed groups showed a dose-dependent acceleration in reaggregation similar to that found following in vivo exposure. In contrast, in vitro exposure to cycloheximide resulted in only a dose-related inhibition of reaggregation. No acceleration in growth rate was seen. Colchicine exposure produced no initial inhibition but appeared to mimic the long-term effects seen with both in vivo and in vitro MeHgCl exposure. These studies suggest that MeHgCl alters cerebellar cell recognition through a complex mechanism initially involving depressed synthesis of specific proteins followed by alterations in microtubules. Both effects may involve a disruption in the arrangement of specific cell surface recognition molecules.