One type of normally occurring isoantibodies in cattle serum, which has not been described before and which corresponds perfectly with the immune anti-Y₂, has been demonstrated. A seasonal variation of normally occurring isoantibodies has been established. In all groups of animals, the highest titres were found to be present during the late summer and the lowest titres during the late winter. There appears to be no difference between sexes and among breeds in this seasonal variation, which is not influenced by feeding. A correlation of climatological factors and naturally occurring isoantibodies was made and it is believed that radiation and temperature may have an influence on the seasonal variation of normal antibodies. An experiment was carried out to exclude any form of light influence on six animals with normally occurring isoantibodies in their serum. In spite of this the seasonal variation in their antibody level was found to be similar to that in a control group exposed to light. It appears that of the climatic factors temperature has the most stimulating effect on the production of antibodies, but that a retardation of about two to three months takes place in the antibody producing systems. Any gradual change in temperature appears to be effective in bringing about an increase or decrease in the level of normally occurring isoantibodies. A corresponding variation of antibodies against Brucella abortus to that of normally occurring isoantibodies could be demonstrated.