The self-association of the penicillin drug, sodium nafcillin monohydrate, in aqueous solution has been examined as a function of temperature and electrolyte concentration. Critical concentrations determined by conductivity measurements in water over the temperature range 288.15 to 313.15 K indicated a minimum critical concentration at 303 K. Thermodynamic parameters of aggregation were derived from the critical concentration data using a form of mass action model modified for application to systems of low aggregation number. Values for the enthalpy of aggregate formation Delta H-m(0) calculated by this method showed that the aggregation became increasingly exothermic with increase of temperature. The predicted Delta H-m(0) at 298.15 K was in good agreement with the value determined experimentally by calorimetry. Critical concentrations and the size and effective charge of aggregates were determined in the presence of added electrolyte (0.05-0.40 mol kg(-1) NaCl) by static light scattering. The interaction between aggregates was interpreted from diffusion data from dynamic light scattering using the DLVO theory. Changes of H-1 NMR chemical shift on aggregation suggested stacking of molecules. Application of mass action theory to the concentration dependence of H-1 NMR chemical shifts confirmed the aggregation number from light scattering.