Whole sera and serum fractions from 24 patients with Felty's syndrome, 42 patients with systemic lupus erythematosus (SLE), and 48 patients with rheumatoid arthritis (RA), as well as 30 patients with miscellaneous acute and chronic disease states, were studied for their effect on numbers of mouse bone marrow colonies grown on soft agar in the presence of human colony stimulating factor. Significant early retardation of mouse bone marrow colony counts was recorded in 87.5 percent of Felty's sera, 43 percent of SLE sera, and 12.5 percent of sera from patients with uncomplicated RA. Forty percent of 30 other control patients with acute or chronic inflammatory diseases also showed this activity. No diminution was noted with any of 40 normal control sera. Degree of marrow colony retardation could be directly correlated to amounts of test serum added. No single serum fraction isolated by ion exchange chromatography, gel filtration, or electrophoresis was identified as solely responsible for marrow growth retardation; however lipoprotein fractions including chylomicrons, LDL and HDL showed inhibiting activity in various sera.