The AA. have carried out researches into the results of the platelets aggregation test (PAT) on 375 people by use of the Born method; they have employed ADP and a commercial available cephalin (Thrombofax, Ortho). In the first case the parameters taken into consideration are: the minimum dose of ADP able to cause a "double wave aggregation", the slope and the maximum increase in light transmission; in the second case the parameters considered are: the latency period, the maximum speed and the maximum amplitude. The normal values, in 75 persons, are: --6 +/- 2 X 10(-7)M (min dose wor "double wave"); --15,5 +/- 4,9 UT/min (slope); -- 46,3 +/- 6,8 UT (max increase in light transmission); regarding ADP aggregation. --90 +/- 40 sec (latency period); -- 30 +/- 15 UT/min (max speed); -- 52 +/- 4,63 UT (max amplitude); relating to Thrombofax aggregation. Analyses of PAT, by parallel using both aggregating agents, have been made on 260 women undergoina an oestrogen-progestative therapy, on 25 daibetic subjects and on 15 subjects undergoing an anti-aggregation oral therapy (Ageroplas, Serono, Roma). The AA. stress on the validity of ADP as an aggregation agent in routine tests, while Trombofax has revealed itself to be not very sensitive in all the cases with hyperaggregation. However it is to be noted that use of cephalin results interesting in tests made on subjects undergoing an anti-aggregation therapy, as the PAT parameters seen proportional to the quantity of the drug given to the patient, which is just the opposite of what happens with ADP.