To investigate whether growth factors derived from T cells in psoriatic lesions are able to stimulate keratinocyte growth, T-cell lines were initiated from lesional psoriasis skin and cloned by limiting dilution. Eight clones with good proliferative capacity out of 40 clones from one patient were stimulated. After 24 h, the conditioned medium was harvested and the growth modulatory effect of the conditioned medium on keratinocytes was assessed. Seven of the eight T-cell clones stimulated keratinocyte growth to an extent ranging from 22% +/- 19 to 64% +/- 9 (mean +/- SD of three experiments) of maximal inducible keratinocyte growth, and one T-cell clone had no effect (-5% +/- 2) on keratinocyte growth. Keratinocyte growth was also induced by T-cell clones obtained from two other patients. Several cytokines were tested in this system to determine which T-cell growth factor may induce the keratinocyte growth. None of the cytokines interferon-g, transforming growth factor-beta, interleukin (IL)-2, IL-3, IL-4, IL-6, IL-8, or granulocyte-macrophage colony stimulating factor alone was found to possibly be responsible for the T-cell-induced keratinocyte growth. Thus the nature of the T-cell keratinocyte growth-promoting stimulus remains to be elucidated.