The cellular and humoral immune responses of 41 hydatidosis patients, 12 healthy uninfected individuals, and seven patients with other parasitic diseases were determined using serologic and lymphoproliferative assays (LPAs), respectively. Echinococcal antigens were obtained by gel filtration of crude hydatid cyst fluid (HCF) on a Sephadex G-200 column. The fractions contained either a mixture of antigens A plus B or antigen B alone that was further enriched by boiling. All the hydatidosis patients responded positively by LPA to either crude or purified echinococcal antigens: 95% of them responded to either crude HCF, or a mixture of antigens A plus B and 83% to antigen B alone. The degree of the response to crude HCF (mean stimulation index [SI] = 75.3) was higher than that of purified antigens (SI = 39.1 for a mixture of antigens A plus B and SI = 36.9 for antigen B alone). No positive LPA response was obtained with the control groups. Serologic examinations showed that 78% of cases were positive by immunoblot, 73% by indirect hemagglutination, and 46% by immunoelectrophoresis. No correlation between the degree of cellular and humoral responsiveness to both crude and purified echinococcal antigens was observed. Nine of the 41 patients examined who were serologically negative also developed a high lymphoproliferative (LP) response to either crude or purified echinococcal antigens. The LP response remained positive over a long period after successful treatment. No relationship was observed between the results of treatment and the LP response. The present study indicates that the LPA could be used as an additional tool for the diagnosis of hydatid disease, particularly in seronegative cases, although it is unsuitable for effective monitoring treatment/surgery.