Neurotrophic activities in human serum and post-mortem muscle and spinal cord of possible relevance to pathophysiological mechanisms in amyotrophic lateral sclerosis (ALS) were studied. Tests included in vitro assays for nerve fibre outgrowth from sympathetic ganglia and for survival promotion of dissociated ciliary neurons, both types of neurons, of chicken embryo origin. Extracts of postmortem biceps muscle promoted survival of ciliary neurons in a dose-dependent manner. Half-maximum effect was found at a protein concentration of about 450 micrograms/ml for both ALS and control muscle. Ventral horn extracts were about 5 times as efficient as muscle in promoting neuron survival, again with no differences seen between control and ALS samples. Sera from patients suffering from ALS as well as normal sera did not enhance survival of ciliary neurons to any considerable extent, nor did they induce fibre outgrowth from sympathetic ganglia. Both groups of sera, if present above 5% in the medium, suppressed fibre outgrowth induced by added nerve growth factor (NGF). Sera from some of the ALS patients impaired survival in dissociated ciliary neurons supported by a trophic activity in choroid extract. The results do not indicate major neurotrophic deficits as the cause of ALS disease but suggest that a neurotoxic mechanism may be involved.