Studies were made of a group of enzymes derived from the pineapple and available in a concentrated form as bromelin. Isotonic solutions of this material, at a concentration of 0.5%,buffered to pH 5.5, induced rapid agglutination when mixed with sera containing erythrocyte antibodies and erythrocytes. The prepared reagent was stable at 4[degree]C and was used to devise an immediate and a 15-minute bromelin test. Comparison with an indirect Coombs'' test and a papain-cysteine test revealed that the bromelin tests in general were more sensitive, with sharp end points. The bromelin tests were found to have a wide range of activity and were used to detect antibodies directed against the ABO, Rh-hr, MNS, P, Lewis, Kell, Kidd, Duffy, Vel, and Sutter blood groups. The same reagent was utilized to detect auto-antibodies coating erythrocytes in 11 cases of acquired hemolytic anemia. An immediate and a 15-minute bromelin test were devised and found to be effective in all cases.