To test the hypothesis that a serum factor may be toxic to motoneurons in Motoneuron disease (MND), we evaluated neuronal survival in dissociated cultures from chick ciliary ganglia neurons in which MND or control sera were applied. No neurotoxic activity was found when comparing neuronal survival at 1 and 3 days in MND and control group. To examine the influence of the clinical MND status on neuronal survival, we used the following parameters: age at onset, sex, months from onset of disease to sera sampling, MND form, rhythm of progression, and scoring with Appel scale. No effect was noted for any of these factors. Sera from the same patient obtained at different clinical stages did not allow any conclusion of a fixed behavior for the same patient. Our findings argue against the existence of a cytotoxic factor in sera from MND patients.