It was previously speculated that TGFβ acts as an 'activator'-molecule in chondrogenic pattern formation in the limb micromass culture system, but its precise role and relationship with the cell sorting phenomenon have not been properly studied. In the present study, we examined whether the TGFβ2 molecule satisfies the necessary conditions for an 'activator'-molecule in the reaction-diffusion model. Firstly, we showed that TGFβ2 became localized at chondrogenic sites during the establishment of a chondrogenic pattern, and exogenous TGFβ2 promoted chondrogenesis when added in the culture medium. Secondly, TGFβ2 protein was shown to promote the production of its own mRNA after 3 hr, indicating that a positive feedback mechanism exists which may be responsible for the emergence of the chondrogenic pattern. We then found that when locally applied with beads, TGFβ2 suppressed chondrogenesis around the beads, indicating it induces the lateral inhibitory mechanism, which is a key element for the formation of the periodic pattern. We also examined the possible effects of TGFβ2 on the cell sorting phenomenon and found that TGFβ2 exerts differential chemotactic activity on proximal and distal mesenchyme cells of the limb bud, and at very early phases of differentiation TGFβ2 promotes the expression of N-cadherin protein which is known to be involved in pattern formation in this culture system. These findings suggest that TGFβ2 acts as an 'activator'-like molecule in chondrogenic pattern formation in vitro, and is possibly responsible for the cell sorting phenomenon. (C) 2000 Wiley- Liss, Inc.