Twenty one chromium containing materials were examined for carcinogenic activity in a two year study using an intrabronchial pellet implantation system whereby pellets loaded with test material were surgically implanted into the lower left bronchus of rats. The principal aim of the study was to extend our knowledge of the carcinogenic potential of chromium compounds and, in particular, chromates (Cr6+). A statistically significant incidence of treatment related lung tumours was found with some sparingly soluble chromate materials. All tumours were large keratinizing squamous carcinomas of the left lung, except for a single left lung adenocarcinoma and two left lung anaplastic carcinomas. No bronchial carcinomas (0/100) were seen in the negative control group (blank pellet loaded with cholesterol), whereas bronchial carcinomas (22/48 and 25/100) occurred in the two positive control groups which received pellets loaded with 20-methylcholanthrene and calcium chromate respectively. Among the 20 test materials, only three groups gave statistically significant numbers of bronchial carcinomas. Two of these were groups receiving different samples of strontium chromate which gave 43/99 and 62/99 tumours. The third group, zinc chromate (low solubility), gave 5/100 bronchial carcinomas. A further zinc chromate group (Norge composition) produced 3/100 bronchial carcinomas which was not statistically significant. A few lung tumours were observed in other test groups.