The use of counter-immunoelectrophoresis (CIEP) in the diagnosis of Johne's disease was investigated in flocks of sheep in Iran and Libya. In 50 sheep with no history of paratuberculosis none had antibodies to Mycobacterium paratuberculosis. Animals vaccinated as neonates were either negative or had reciprocal titres (RT) of less than 4 at 18 months of age unless exposed to infection. Among 105 non-vaccinated adults in an infected flock 91% had antibodies. Twenty-one animals that either died or were slaughtered were autopsied. Eight animals with RT of 8 or above and one with RT of 4 had gross lesions with demonstrable acid-fast organisms. Two animals with RT of 2 had clinical disease while three with RT of 4 and one with RT of 2 had histopathological lesions only. All animals found negative for M. paratuberculosis infection had RT of 2 or below. The critical interpretation of the test is discussed.