d-Aminolevulinate dehydratase (d-ALA-D) catalyses the condensation of two aminolevulinic acid molecules with the formation of porphobilinogen, which is a heme precursor. d-ALA-D is a sulfhydryl-containing enzyme that is inhibited by a variety of oxidizing agents. The present study investigated the effect of bis and tris-selenide alkene derivatives, a class of organoselenium compounds, on human blood d-ALA-D. To investigate a possible involvement of cysteinyl groups in enzyme inhibition induced by bis and tris-selenide alkene derivatives, the effect of thiol reducing agents was examined. d-ALA-D activity was assayed according to the method of Berlin and Schaller (1974). Data were analyzed statistically by one-way ANOVA, followed by Duncan’s Multiple Range Test when appropriate. Bis-selenide alkene 1a (R= 4-MeOC₆H₄) in concentrations higher than 200 mM significantly inhibited erythrocytic d-ALA-D, the maximal inhibitory effect (35%) was observed at 600 mM. Bis-selenide alkenes 1b (R= 4-ClC₆H₄)  and 1c (R= 2,4,6-Me₃C₆H₂) did not alter d-ALA-D activity. Tris-selenide alkene 2a (R= C₆H₅), 2b (R= 4-MeOC₆H₄) and 2c (R= 4-ClC₆H₄) inhibited d-ALA-D from human erythrocytes. At 600 mM, tris-selenide alkenes 2a, 2b and 2c consistently inhibited d-ALA-D activity (65%, 35% and 45%). Dithiothreitol, a hydrophobic SH-reducing agent, was able to protect inhibited d-ALA-D. From these findings we suggest that bis- and tris-selenide alkenes inhibited erythrocytic human d-ALA-D by an interaction with the sulphydryl groups essential for the enzyme activity.