Three metabolizing systems (rat, heterologously expressed CYP3A4 and human liver) were used to evaluate 12 analogues of dapsone (4,4'diaminodiphenylsulphone) in-vitro. Methaemoglobin formation in a two-compartment and cytotoxicity in a single-compartment model were studied using human erythrocytes and neutrophils, respectively, as target cells. In the two-compartment system using rat microsomes as a generating system and methaemoglobin as an endpoint, the least potent methaemoglobin formers tested were the 2-methyl-4-propylamino (AXDD14), 2-hydroxy-4-4'amino (ABDD5) derivatives and a sulphone/trimethoprim derivative (K-130). Dapsone itself, a 2-methoxy-4-ethylamino (W10) and a 2-hydroxyl-4-ethylamino compound (ABDD39) were the most toxic. In the single-compartment cytotoxicity test using rat microsomes, AXDD14 was again among the least toxic, as was a 2-methyl 4-cyclopentyl derivative (AXDD17) and surprisingly ABDD39. The most cytotoxic compounds again included dapsone itself as well as two 2-trifluoromethyl derivatives. The only significant methaemoglobin formation and cytotoxicity shown with the heterologously expressed human CYP 3A4 was with AXDD14, which was extensively activated. Interestingly, metabolism of dapsone was low using the expressed CYP 3A4. In the two-compartment system using human liver microsomes, AXDD14, K-130 and ABDD5 were oxidized to a significantly lesser extent compared with dapsone and these preliminary findings indicate that future development of these compounds may be worthwhile.