The costimulatory factors CD28, CD80, CD86 and CD152 needed to start and turn off an immune response are present as membrane receptors and soluble proteins. There was no difference in the serum levels of soluble costimulatory molecules in 153 healthy controls and 118 patients with myasthenia gravis. However, we could confirm that the soluble forms of ICAM-1 and CD25 were increased in patients. The concentrations of the soluble costimulatory proteins seemed to be rather constant in individual patients, despite changes in clinical presentation. Thus, the soluble costimulatory factors do not seem to constitute reliable markers for disease activity in myasthenia gravis.