OBJECTIVE To evaluate skin absorption of cobalt powder in an in vitro system. Experiments with volunteers show that cobalt powder can permeate through the skin, but there are no data with regard to the mechanism or the amount of permeation. METHODS Skin permeation was calculated by the Franz diffusion cell method with human skin. A physiological solution was used as receiving phase and the cobalt powder was dispersed in synthetic sweat. The amount of metal passing through the skin was analysed by electro-thermal atomic absorption spectrometry (ETAAS). Parallel polarographic analysis (differential pulse polarography-DPP) allowed evaluation of cobalt present as ions (Co(2+)) in donor and receiving phases. Measurements of cobalt skin content were also performed. RESULTS Evaluation of metal in the receiving phase allowed us to demonstrate the permeation of cobalt through the skin. Steady-state flow of percutaneous cobalt permeation was calculated as 0.0123+/-0.0054 microg cm(-2) h(-1), with a lag time of 1.55+/-0.71 h. CONCLUSIONS The experiments show that cobalt powder can pass through the skin when applied as a dispersion in synthetic sweat, oxidising metallic cobalt into ions, which permeate the skin. These experiments show for the first time how cobalt can permeate the skin.