The widely accepted hypothesis of vanadate action on cells postulates that this ion inhibits protein phosphatase(s) that dephosphorylates protein phosphotyrosine residues. This inhibition causes tyrosine hyperphosphorylation of cell proteins followed by changes in physiological action of phosphoproteins resulting in stimulation of cell proliferation, expression of protooncogenes, and transient cell transformation. We have found that treatment of human ovary carcinoma (CaOv) cells with vanadate causes the increase in total protein phosphorylation from 1.5- to 2.0-fold whereas the ratio between phosphoserine, phosphothreonine, and phosphotyrosine content remains unchanged. At the same time, enhancement of c-myc gene expression (not c-fos) was observed. Hence, the increase in the ratio of phosphotyrosine to phosphoserine and phosphothreonine is not an obligatory intermediate stage before vanadate-dependent activation of c-myc expression.