The transforming growth factor-β₁ was known as having the most important influence on chondrocytes among various growth factors, being abundant in articular chondrocytes and osteocytes. We performed in vitro monolayer cultures of human articular chondrocytes from normal and osteoarthritic patients and studied the transforming growth factor-β₁ responsiveness of those chondrocytes. The cell-growth curve indicated that the primary osteoarthritic chondrocyte culture with transforming growth factor-β₁ showed a more rapid growth pattern than normal chondrocytes with or without TGF-β₁ and osteoarthritic chondrocytes without TGF-β₁. The osteoarthritic group showed a sharp decline in growth pattern with subsequent culture. The shape of osteoarthritic chondrocytes was bigger and more bizarre compared to those of normal chondrocytes. With subsequent culture, this change became prominent. The transforming growth factor-β₁ increased the [³H]-TdR uptake in each group. The phenotypes of chondrocytes were more clearly expressed in the normal group. The chondrocytes lost their phenotype (production of collagen type II) following subculture in each group. The transforming growth factor-β₁ could not inhibit or delay the dedifferentiation process (loss of phenotype).