I have examined the effects on cells in the developing eye of over-expressing the argos gene. Transgenic flies carrying argos expressed under hsp70 and sevenless control sequences were analysed. All cell types in the developing eye (except bristles) are sensitive to argos concentration: over-expression leads to too few cells forming, the opposite phenotype to that seen in argos loss-of-function mutants. This effect was only seen with HS-argos flies: sev-argos flies, which over-express the protein at a lower level are not affected, suggesting that a considerable over-expression is required to disrupt cell fate. However, sev-argos is able to rescue argos eye mutations completely, indicating that the normal expression pattern is not critical for wild-type eye development. By transfecting argos into tissue culture cells, I show that the protein is secreted in a soluble form.