The authors describe a complement fixation technique on microtitration plates, using an antigen prepared from myxomas induced in rabbits. Compared with indirect immunofluorescence this technique was less cumbersome, more economical, easier to read and (as a conventional procedure) applicable in all laboratories. Results obtained with 165 serum samples tested by both methods showed good correlation and a specificity at least equal to that of indirect immunofluorescence. Taking into account its lower sensitivity, the positive threshold value for complement fixation under the described experimental conditions was a dilution of 1:4 (H50).