The water protein spin-lattice relaxation time (T1) was measured in suspensions of human peripheral blood mononuclear cells (PBMC), uninfected or infected with type 1 herpes simplex virus, human cytomegalovirus and rubella virus. In the infected samples, T1 enhancements, which linearly depend on virus concentration, were observed. This T1 increase can be related to the early changes induced by the virus adsorption of the cells, not always confirmed by virus-induced cytopathic effect (CPE) in cocultures of infected PBMC and other sensitive cells. Compared with other conventional virological techniques, this NMR method seems to be rapid and sensitive. The NMR response was reproducible and specific, since neutralization of the viral infection by homologous antisera consistently matched the neutralization of the virus-induced NMR effects. These observations suggest that fast and sensitive 1H-NMR relaxation techniques can be implemented in virological diagnosis directly on pathological materials.