Purified cultures of neonatal rat cortex astrocytes can be used to measure astrocyte mitogenic activity in human brain extracts. A microassay permits the strict comparison of multiple samples. Human brain mitogenic activity was 2 to 4-fold that found in comparable extracts from rat brain. A 1.3-fold increase in specific mitogenic activity was observed in extracts from Alzheimer's disease cortex when compared to normal aged or adult tissue. This small increase was due to the lower amount of extractable protein in Alzheimer's tissue.