Abstract Objective To investigate the relation between lesions and cellular immune response-based tests in sheep with naturally acquired paratuberculosis and to evaluate efficacy of an interferon-γ (IFN-γ) assay and the intradermal skin test. Animals 110 and 88 adult sheep were used to evaluate the IFN-γ assay and skin test, respectively. Procedure Before euthanasia, heparinized blood was collected and incubated with avian purified protein derivative (PPD). Interferon-γ production was assessed, using a commercial enzyme immunoassay. Skin testing in the caudal fold also was performed, using avian PPD. Sensitivity and specificity of these techniques were evaluated, using histopathologic findings as reference. Results Of the sheep examined, 61 (55.4%) had lesions that were classified in 5 categories, according to location, extension, cellular types, and presence of acid-fast bacilli. Regarding cellular immunity test results, 55.4% of sheep with and 6.1% without pathologic changes had positive results of the IFN-γ assay. Skin test sensitivity (55.6%) and specificity (100%) were slightly greater. Conclusion Close association between pathologic findings and cellular immune response was apparent. Most sheep with focal (tuberculoid) and some with diffuse lesions (borderline tuberculoid), all with scant or no acid-fast bacilli in the intestine, had positive results of the IFN-γ assay and skin test. Most sheep with diffuse lesions and high numbers of bacilli (borderline lepromatous) were test negative. Clinical Relevance Cellular immune response-based and serologic tests are useful tools for seeking evidence of paratuberculosis. Each technique detects different types of immune-reacting sheep, and their simultaneous use may detect most infected animals. (Am J Vet Res 1999;60:123–127)