Epidemiologic evidence has recently identified an association between an endemic outbreak of pleural and peritoneal mesothelioma in the Urgup region of Turkey and exposure to zeolite fibres. This malignancy is usually associated with exposure to asbestos dusts whose mineralogical characteristics differ from those of zeolites. The present study further defines the in vitro biologic activity of erionite, a common zeolite fibre found in the Urgup region of Turkey. Both erionite and crocidolite asbestos fibres were clastogenic in synchronous Chinese hamster ovary (CHO) fibroblasts. Both fibres also altered CHO ploidy. Erionite, unlike crocidolite or Min-U-Sil quartz, caused a slight increase in sister chromatid exchanges in synchronous CHO cells. Neither erionite nor crocidolite was mutagenic in a human lymphoblastoid cell line. Erionite fibres thus produced in vitro cytogenic changes similar to those caused by asbestiform mineral dusts and, like asbestos fibres, did not induce mutations in human lymphoblastoid cells.