Age-related circulating CSF-1 levels in untreated SJL/J mice. Female SJL/J mice were bled repeatedly at the ages of 1, 2, 3, 4, 6, 8, 10, and 12 months and the concentration of CSF-1 in the sera was determined by RIA (•). Up to age 8 months, CSF-1 levels reflect the mean ± SD serum CSF-1 concentrations from 20 to 24 disease-free mice. Commencing at 10 months, approximately one third of the mice spontaneously developed a B-cell neoplasm (RCN B, ▪). Values in parentheses indicate the number of mice at each time point. *CSF-1 levels significantly different (Student's t-test, P < .05) from the levels at 3 months of age.