In the present study the effect of different dialysis membrane materials on in vitro β<sub>2</sub>-microglobulin (β<sub>2</sub>M) release by human lymphocytes was investigated. Lymphocytes were isolated from 11 long-term haemodialysis patients and 9 healthy controls. Cells were cultured either on flat sheet membranes made from Cuprophan, Hemophan and polyacrylonitrile or on polystyrol. β<sub>2</sub>M concentrations in the supernatant were measured at 3 and 7 days of culture, respectively. The β<sub>2</sub>M release from lymphocytes obtained from uraemic patients was almost identical with the release from healthy subjects. In the presence of all three membranes the release of β<sub>2</sub>M was less of what was produced on polystyrol. This held true for lymphocytes isolated from both healthy and uraemic subjects. As for the three membrane materials, the release of β<sub>2</sub>M into the supernatants was statistically not different when the adsorptive capacity of polyacrylonitrile was taken into account. However, there was a tendency for Cuprophan to exert the strongest inhibition, while Hemophan and polyacrylonitrile reduced β<sub>2</sub>M release to a lesser degree. Based on these data it seems that the increase in serum β<sub>2</sub>M which has been observed during dialysis with Cuprophan membranes is not caused by direct interaction of lymphocytes with Cuprophan membranes.