70 patients were randomized to 3 groups. Group I : patients received 5,000 IU calciparineR subcutaneously given 2h before operation, twice a day for 3 days and then, doses were adjusted ty heparin levels. Groups II and III : 2,500 IU Kabi 2165 were given instead of calciparineR, but in group III, a single daily dose of 5,000 IU Kabi 2165 was administered from the third day. Fibrino-peptide A (FPA), FPA generation, D-dimers were measured in plasma by ELISA methods. Anti Xa activity, Xa generation, heparin cofactor II (HC II), antithrombin III (AT III) were assayed in plasma using amidolytic methods. Serial measurements were done : pre-operatively, 2 and 8h after the first heparin injection and on the 7th post-operative day, before heparin administration. Pre-operatively, the mean level of D-dimers was higher (p<0.05) in 9 patients with bleeding complications after surgery (0.83 ± 0.57 pg/ml) compared to other patients (0.42 ± 0.33 pg/ml) 2h after heparin injection : 1) Xa activity was lower (p<0.001) in group I (<0.05 IU/ml) than in groups II and III (0.137 ± 0.1 IU/ml) ; 2) inhibition of Xa generation decreased (p<0.001) in group I (11.7 ± 8%) compared to that in groups II and III (25.8 ± 13%) ; 3) inhibition of FPA generation was lower (p<0.05) in group I (27.1 ± 32.2%) than in groups II and III (45.7 ± 29.2%) ; 4) diminution of D-dimers under heparin (0.37 ± 0.28 pg/ml) was only significant (p<0.05) in groups II and III. 8h after the first heparin injection, in all groups : 1) D-dimers increased (p<0.001) compared to pre-operative values ; 2) AT III and HC II diminished (p<0.001) ; 3) no anti Xa activity and no inhibition of Xa and FPA generation were found. On the 7th day : 1) 4 patiertts with deep vein thrombosis had higher FPA than other patients ; 2) AT III but not HC II fell in group I (p<0.05) ; 3) no anti Xa activity and no inhibition of Xa and FPA generation were observed in all groups. In conclusion : 1) although clinical results are similar in the 3 groups, Xa activity, Xa generation, FPA generation change differently according to the heparin used ; 2) the occurence of deep vein thrombosis is followed by an FPA increase and bleeding complications are correlated with a high level of pre-operative D-dimers.