The affinity of doxycycline for crystalline plasma albumin fraction V, originating from sheep, dogs, cats, cows, pigs and humans, was evaluated by means of double-reciprocal and Scatchard plots. Mathematical modelling and weighted least-squares non-linear regression analysis of each Scatchard plot identified one binding component characterized by one high affinity binding site, and a second component attributed to non-specific binding to albumin. Association constants for this binding site ranged from 38 471 ± 13 369 (SEM) l/mol for the interaction of doxycycline with ovine albumin to 6405 ± 2375 l/mol for the interaction of doxycycline with human albumin. Statistical evaluation of the results suggested slight species-related differences in the values of association constants. Diphenylhydantoin, phenobarbital or carbamazepine did not displace doxycycline from binding sites on bovine albumin.