Antimicrobial agents often generate doubts concerning their therapeutic effects; it is not infrequent to find differences in the clinical effectiveness among products of the same active principle made by different laboratories. It is, therefore, necessary to carry out studies as to the reasons of such variations in order to reduce or eliminate differences in the therapeutic products. We evaluated the fundamental characteristics (external form, internal structure, and physical and chemical identification) of ampicillin powder using a trade name product (powder in capsules), a generic one (powder in capsules), as well as different crystalline forms of the raw material (also in powder), obtained by recrystallization in different solvents at room temperature. After determining the fundamental properties of the aforementioned solids, they were compared on the basis of the results obtained in their derivative properties such as the dissolution profiles analyzed by means of the difference and similarity factors (f1, f2), and the antimicrobial effectiveness indexes by means of the minimal inhibitory concentration (MIC).