The effect of fosfomycin on the thrombocytal aggregation induced by ADP was studied. Plasma rich in thrombocytes was used. It was obtained from blood donors. Fosfomycin was used in concentrations of 1.3 X 10–3; 1.3 X 10–2, and 1.3 X 10–1 m and the ADP in concentrations of 1.17, 2.34 and 3.51 X 10–6 m. Two experimental groups were used: the first group was made up of five experiments, with an incubation time for the fosfomycin with the thrombocytes of 2 min; the second experimental group was made up of five experiments whose incubation times were 5 and 10 min. Fosfomycin inhibits the thrombocytal aggregation which is induced by ADP. This inhibition (1) is larger, the smaller the concentration of ADP, (2) for a single concentration of ADP it is proportional to the concentration of fosfomycin that is used, and (3) there are no significant differences according to the time of incubation. This thrombocytal anti-aggregating activity is related to the presence of a phosphonic group in the molecule of fos