Four factors, suspected of influencing the final results of agglutinin-absorption tests used in the diagnosis and classification of leptospires, were investigated in comparative studies: (1) the time required for adequate absorption, (2) the quantitative relationship between antibody titres and amounts of antigen needed for absorption, (3) the possible effect of the Danysz phenomenon, and (4) the absorptive potency of live and formol-treated antigen.It was found that a 90-minute absorption time was adequate and that with increasing amounts of antigen, titres were continuously reduced, indicating a certain degree of non-specific absorption. The Danysz phenomenon was found to occur in leptospiral serology and the addition of antigen to serum in 3 equal parts at 10-minute intervals is recommended.The titres of sera absorbed with formol-treated antigen were often found to be lower than titres of sera absorbed with the same amounts of live antigen and some damaging effect of formol on antibody is suspected.