The potential of albumin (bovine serum) microspheres as delivery systems for haematoporphyrin and/or its zinc derivative was probed at the molecular level. The microspheres, prepared by heat denaturation, were stable to detergents and organic solvents but could be degraded by proteolytic enzymes. Drug loading was either by encapsulation during microsphere formation or by binding to preformed microspheres. The efficiency of encapsulation was found to depend on the initial drug/protein ratio with a saturable pattern. Binding of both drugs, or each alone, indicated the presence of two types of affinities of each drug to the microspheres. The leakage (release) of drug(s) from the microspheres into the aqueous medium over a period of several hours showed biphasic behaviour for each type of preparation, bound and encapsulated, for one or both drugs A model assuming the microsphere-bound drug is distributed into two pools, differing in their drug-microsphere affinities, with no interference between the two drugs if both were bound, was found to account well for the equilibria data. Binding constants for respective pools were found to be similar for both drugs, of the order of 0.10 and 2 × 10-4ml/μg protein for the high and low affinity pools, respectively. The low affinity pools had the higher drug population. The two-pool model also fits well with the kinetic data, the pools differing in the rates of release. Rate constants for both drugs were found to be similar, of the order of 0.3 per min and 0.025 per hour for the fast and slow releasing pools, respectively. The population of the slow-releasing pool was higher, the high-affinity pool being the fast-releasing one, the low-affinity pool being the slow-releasing one. Implications for the microsphere-mediated delivery of these drugs in vivo are discussed.