Cell-mediated immunity was studied in 21 parasite-naive dogs by transformation of peripheral lymphocytes with phytohemagglutinin (PHA), concanavalin A (Con A), or hydatid fluid protein (HFP) at 29 days of a primary infection with Echinococcus granulosus, and by delayed cutaneous hypersensitivity to HFP at 36 days of the infection. Infection depressed significantly the ability of unstimulated cells to proliferate but enhanced the response to Con A. The lymphocytes of four dogs reacted to HFP significantly above the average, and one dog exhibited cutaneous reactivity to 0.2 micrograms of HFP. Five or 1 but not 0.2 micrograms of HFP produced non-specific skin reactions in parasite-naive dogs. The 7 dogs with the highest reactivity to PHA or Con A had significantly fewer parasites than the 7 less reactive dogs. The differences in other parasite characteristics were not significant in the 7 dogs with the greatest or smallest reactivities. Correlation tests showed an inverse correlation of PHA reactivity with establishment and maturation of parasites, of Con A reactivity with growth and maturation, and of HFP with maturation.