The self-association of the penicillins cloxacillin, dicloxacillin, and flucloxacillin in water and in the presence of added electrolyte (0.025-0.40 mol kg(-1) NaCl) at 30 degrees C has been examined by light-scattering and NMR techniques. Inflections in he data from both techniques were observed at a single critical concentration for solutions of cloxacillin and at two critical concentrations for dicloxacillin and flucloxacillin. Aggregation numbers and effective micellar charges were calculated from the static light-scattering data for the stable aggregates formed at the first critical concentration. Application of the valance-generalized light-scattering theory for multicomponent systems to data at concentrations above the second critical concentration provided an estimate of the aggregate size of the associated species present at high solution concentration. The interaction between aggregates was interpreted from diffusion data from dynamic light-scattering using DLVO theory. Micellar properties have been determined by the application of mass action theory to the concentration dependence of H-1 NMR chemical shifts, confirming the results obtained by the light-scattering technique.