An analysis is given of the serological tests performed over a period of 15 months with viral, rickettsial and leptospiral antigens. The analysed material is made up of about 9400 tests performed on 4036 samples of serum obtained from 2430 patients and 536 animals.

The incidence of the various diseases, the distribution of the positive results according to diseases, and the height of the specific titres, as well as the height of the residual titres found in the material, are discussed and analysed in detail.

The procedure for the preliminary screening of the material, as well as the methods used in performing the different serological tests, are described and discussed.

A few examples of possible double infections are quoted, namely atypical virus pneumonia with influenza, and influenza A with B.

An analysis of the results of 1723 tests performed with influenza antigens on 909 samples of sera is presented.

The results obtained from testing about 1000 sera for cold agglutinins and 482 sera for MG agglutinins are discussed in detail.

Some cases of liver affections showed a marked rise in titre both of cold agglutinins and of MG agglutinins.

The serum samples, numbering about 1050, drawn from normal persons or patients suffering from infections other than Q-fever, all gave titres lower than 1/64 when tested with Q-fever antigen, except the sera of four persons who were probably cases of recent inapparent infection.

The geometrical mean of the titres found in sera drawn from acute Q-fever patients between the 29th and 60th days of their illness was 1/355. Of the 500 sera from various animals, 100 from sheep and eight from cows had titres of 1/8 to 1/64 against the Q-fever antigen.

Thirty-four sera have been positive when tested with the soluble antigen of both the epidemic and the murine types of typhus but no serum has given a higher titre with the murine type antigen than with the epidemic type antigen. Of the thirty-four sera tested with both antigens the titres obtained with the epidemic antigen were higher than with the murine antigen in 28.

The results obtained with 417 sera tested with the mumps antigen, and with the 222 sera tested with the lymphocytic choriomeningitis antigen are reported and discussed in detail.

Great individual variation has been observed in the time of appearance and rise and fall of the Paul-Bunell titres. Consequently, it is advised that early and frequent blood samples should be obtained from patients who are suspected to be suffering from infectious mononucleosis. On the basis of the results of 316 Paul-Bunnell tests it is suggested that a titre of 1/20, if preceded or followed by a negative serum sample, should be taken as conclusive evidence of infectious mononucleosis.

3080 human and 164 animal sera have been investigated for the presence of antibodies to various types of leptospirae. In both human and animal sera antibodies have been found most frequently against L. sejroe and L. pomona.

The results of a few tests performed with toxoplasma antigen are mentioned briefly.

We wish to express our gratitude to all our colleagues in Belgrade and other parts of the country who have provided us with information and additional samples of blood when requested, in particular Prof. K. Todorowich of the Infectious Diseases Hospital, Belgrade, and Drs Lj. Vuksich, M. Morelj, B. Arsich, D. Mehl and R. Papo of the Army Medical School, Belgrade.

We wish to thank Dr R. Djorich of the Infectious Diseases Hospital, Belgrade, for the supply of material for testing for atypical virus pneumonia and aseptic meningitis, and Dr Ž. Perishich for help with our cases of infectious mononucleosis. Mrs Martha Milivojevich gave valuable assistance with the complement-fixation tests and with the compilation of material for this report.