AbstractThe effects of cachectin/tumor necrosis factor (TNF) on growth and differentiation of 3T3‐L1 cells were examined. This fibroblastic cell line can be induced to differentiate into a mature cell type having the biochemical and morphological characteristics of normal adipocytes. At various stages of growth and differentiation, 3T3‐L1 cells were exposed to 2.5 × 10−16to 2.5 × 10−8M (4.2 fg/ml to 420 ng/ml = ca. 1.2 × 10−14to 1.2 × 10−6U/ml) recombinant human cachectin/TNF for 24 hr, after which cytotoxicity or differentiation was evaluated. During log‐phase cell growth, cachectin/TNF had no significant effect on cell viability, and the preadipocytic cells were also resistant to the cytotoxic effect of cachectin/TNF at the contact‐inhibited confluent stage. However, when cachectin/TNF was added to the cells during induced differentiation, only 20% of the cells survived. After differentiation into adipocytes, cells regained their resistance to cachectin/TNF‐induced cytotoxicity. Cachectin/TNF also markedly affected the differentiation of 3T3‐L1 cells into adipocytes. When cells in the confluent phase of growth were exposed to cachectin/TNF for 24 hr, their subsequent hormone‐induced differentiation to adipocytes was inhibited. Like cachectin/TNF, IL‐1 also induces suppression of lipoprotein lipase and enhances lipolysis in differentiated 3T3‐L1 adipocytes; however, in contrast to cachectin/TNF, IL‐1 had no effect on the viability or differentiation of pre‐adipocyte 3T3‐L1 cells. These results indicate that the cytotoxic action of cachectin/TNF varies in the same cell type depending on the stage of growth or differentiation. The results also imply that cachectin/TNF may play a normal role in controlling the differentiation of certain types of cells i