A new commercially available CEA immunoassay, using monoclonal antibody, was evaluated for the purpose of routine clinical use. The between-day coefficient of variation was 5.2% at the 6.6 micrograms/l level and 4.1% at the 14.2 micrograms/l level. The results of analysis of sera from 260 patients were compared with the data from an established method with polyclonal antiserum. A good correlation between the two methods was found. When the patients were classified according to groups with known types of carcinomas no systematic differences between the monoclonal and polyclonal modes of assay were apparent. Relatively large differences in serum CEA levels did occur in individual patients necessitating a transition period in changing from one method to the other.