Sera collected from 691 dogs and 231 rodents in South Africa were tested in parallel by the passive haemagglutination (PHA) and enzyme-linked immunosorbent assay (ELISA) for antibodies to the Fraction 1 antigen of the plague bacillus (Yersinia pseudotuberculosis subsp. pestis). There was a high degree of correlation between the tests, both in over-all numbers positive and in serum titres. 42 dog sera were positive by PHA and 41 by ELISA, 40 sera being positive by both tests. However, using ELISA the numbers of non-specific reactors in dog sera were cut by almost two thirds compared to PHA. The results of tests on rodent sera were identical: only one rodent was positive in both tests. The findings suggest that the ELISA test may have advantages over PHA for large scale serological surveillance.