Simple or combined D,T,P and inactivated polio vaccines adsorbed onto calcium phosphate are prepared according to two procedures. Antigens can be dialysed in a sodium phosphate solution and quickly mixed with an equal volume of an equimolar solution of calcium chloride, the pH is adjusted to 6.8-7. Toxoids added to the phosphate solution are in this way entrapped within the 3-dimensional network during the formation of the precipitate. Antigens can also be added to a calcium phosphate gel suspension prepared in advance. Results of animal experiments and of field trials using either combined vaccines or simultaneous immunization with diphtheria, tetanus, pertussis, meningococcal and several viral vaccines: polio, rabies, hepatitis-B, etc., will be presented. Several programs have been studied in developing countries, mainly with the aim at simplifying vaccination campaigns. The efficiency of a two-dose regimen with DT vaccine has been ascertained, this has also been applied to pregnant women. Adsorbed tetanus toxoid was successfully used as a diluent for freeze-dried measles, meningococcal polysaccharide and rabies vaccines. Levels of circulating antibodies and potency of vaccines have been measured by new in vivo and in vitro methods. The choice of detoxified purified toxins is desirable for the preparation of vaccines in order to prevent the occurrence of adverse reactions. Local and generalized reactions have been studied. Adverse Arthus-type reactions have been encountered and related with the presence of high levels of circulating tetanus antibodies.(ABSTRACT TRUNCATED AT 250 WORDS)