Different times and temperatures of the 1st phase incubation of the complement fixation test (CFT) were used. It was found that a similar decrease in complement (C') activity occurred after 2 h at 37 degrees C, after 8 h at 20 degrees C and after 18 h at 4 degrees C. Simultaneously, in the same periods a double increase in standard serum titre at 4 and 20 degrees C was noted in comparison with titre at 37 degrees C. This increase of sensitivity of the CFT at 4 and 20 degrees C appeared to be a simulating one and resulted from the different pattern of reaction in each temperature in the block titration of antigen. The positive reactions in the range from the lowest to the highest dilutions of antigen were appearing and increasing and then disappearing in sequence at 4, 20 and 37 degrees C, creating the typical C' fixation area for each temperature. Similar results were obtained in four different C' fixation techniques, most frequently used in diagnosis of animal brucellosis. On the basis of our own examination results and literature data the warm and short C' fixation (37 degrees C, 30 min) was chosen in the standard technique of the CFT used in routine diagnostic survey for animal brucellosis in Poland.