Effects on chromosomes were studied in human lymphocytes in vitro as a result of treatment by coumarin derivatives in the dark. This paper presents investigations with the linear furocoumarins 8-methoxypsoralen (8-MOP; xanthotoxin) and 8-isoamylenoxypsoralen (8-IOP; imperatorin), the angular furocoumarin 5′-methylangelicin (5′-MA), and, in addition, the simple coumarin 5,7-dimethoxycoumarin (5,7-DMC). 8-MOP and 8-IOP influenced the growth of lymphocytes basically in the same way, different from that of the angular derivative; 5,7-DMC had no effect compared to the control. We found a slight but reproducible increase in sister chromatid exchange (SCE) frequency equivalent to 3,7 induced SCE/metaphase with 8-MOP in the dark. 5′-MA and 8-IOP had no significant effect on SCE frequency under these conditions. 5,7-DMC resulted in 1,9 induced SCE/metaphase. 8-MOP also revealed a slight clastogenic activity as 8-IOP does. The chromosome damaging effect was expressed mainly in a doubling of the break rate. Relations between structure and activity of the tested compounds as between both types of cytogenetic effects, SCE and chromosome aberrations, are discussed.