Low-molecular-weight heparin (LMWH) derivatives differ among one another and from unfractioned heparin (UFH) in their half-life as well as in their mode of action. Our aim was to find out, whether the plasma activity levels and their time course after subcutaneous low-dose application of LMWH or UFH in the experimental rabbit are comparable to those in humans. Fairly reasonable standard curves for each LMWH preparation were obtained by using geometrical dilution series of spiked rabbit pool plasma and by applying the methods of Bartl et al. (anti-IIa) and Holmer et al. (anti-Xa). The latter method was slightly modified in order to achieve discernable photometric absorptions for activity levels less than 0.05 U/ml. Rabbits under intravenous general anesthesia received an anterior neck incision. The right external jugular vein was dissected free. A catheter was inserted, advanced into the upper caval vein and kept patent by continuous infusion of 10 ml/h of normal saline. Blood samples of 4 ml were collected in syringes containing 1 ml sodium citrate, before and 1, 2, 4 and 8 h after subcutaneous injection of a LMWH derivative or UFH. The samples were centrifuged and frozen to -30 degrees C. For the estimation of heparin-like anti-IIa and anti-Xa activities the samples were thawed and treated in the same manner as the samples from which the standard curves had been derived.(ABSTRACT TRUNCATED AT 250 WORDS)