Effect of rhIL-6 on IL-6 production by HGF F-CL cells. Various amounts of IL-6 were added to confluent cultures of F-CL fibroblasts and incubated for 1, 2, 4, 8, 24, or 48 h. IL-6 protein in the culture supernatant was determined by ELISA. The values are the means of duplicate determinations. The variation associated with each point is within 1 SD of the mean.