BACKGROUND Antibodies of the Knops system have been referred to as nonneutralizable because they cannot be inhibited with serum, saliva, or urine. Because the Knops system antigens have been located on complement receptor 1 (CR1), the question of whether the antibodies could be neutralized with soluble CR1 (sCR1) produced by recombinant DNA techniques was studied. METHODS First, radiolabeled immunoprecipitation techniques were used to test sCR1 for the expression of the high-incidence Knops system antigens. Then, a total of 45 antibodies were neutralized with sCR1, including the following: one each of anti-Cr(a), -Dr(a), -Do(b), -Hy, -Ge, -Jr(a), -Sc1, -Jk(a), -Cs(a), and -Kp(b); two each of anti-Lu(b), -Yt(a), and -JMH; three each of anti-McC(a), -Rg, and -Sl(a); and four each of anti-Ch, -Kn(a), -Yk(a), -Kn/McC. In addition, two examples of anti-Kn(a) + K, one example of anti-Sl(a) + K + Fy(a), and one example of anti-Yk(a) + E were tested. The sCR1 was added to each test serum and 6-percent albumin was added to the control; this was followed by neutralization incubation for 5 minutes at 25 degrees C. The antibody samples were then tested by a low-ionic-strength solution, anti-human globulin technique. RESULTS The sCR1 expressed Kn(a), McC(a), Sl,a and Yk(a). All Knops system antibodies (n = 22) were neutralized by the sCR1, but none of the other 23 alloantibodies decreased in reactivity. The samples containing antibodies of two specificities showed inhibition of the Knops system antibody but not of the second antibody. CONCLUSIONS This neutralization method, in which recombinant protein is used, provides an expedient and definitive method of identifying Knops system antibodies.