Summary.-It has been reported that lymphocytes from cancer patients give positive responses to PPD, myelin basic protein, tumour basic protein, and certain histone fractions in the MEM test. The underlying mechanisms of the MEM test are poorly understood, but it is widely assumed that it detects immunological sensitization to specific antigenic determinants. The cross-reactivity experienced is interpreted as indicating shared antigenicity. Since all the stimulatory proteins are strongly basic we investigated an alternative explanation that responsiveness is a function of electrical charge by comparing the known stimulatory proteins in the MEM test with two others of similar basicity: lysozyme and cytochrome-C. We obtained highly significant stimulation with PPD, tryptophane peptide of myelin, and tumour basic protein using Mantoux + cancer patients, but found no response to other basic proteins. We failed to confirm the reported activity of histone F2a. Our results indicate that basicity alone is insufficient to elicit response, and strengthens the concept that the MEM test is measuring sensitization to the determinants shared by myelin and tumour basic protein.