A novel technique for the isolation and enrichment of human natural killer (NK) cells from peripheral blood mononuclear cells (MNC) is described. Negative selection of MNC with the lectin from Erythrina cristagalli (ECA), whether by panning or agglutination in solution, resulted in a population of lymphocytes (5-20% of original MNC) highly enriched in cells exhibiting NK function. This enrichment was evident by a significant increase (range of 3-50-fold) in cells with large granular lymphocyte (LGL) morphology, K562 tumor-binding cells, cytotoxic activity, and cells expressing NK phenotypic markers (Leu 11+, OKM1+). Analysis of the cytolytic specificity of the cells demonstrated that the lytic spectrum was typical of endogenous NK. The effector cells were responsive to augmentation of cytotoxic potential by lymphokines (IL-2, IFN alpha, and IFN gamma) and capable of antibody-dependent cell-mediated cytotoxicity (ADCC). ECA-negative [ECA(-)] cells were equivalent to NK isolated by Percoll gradient fractionation. NK heterogeneity was demonstrated by the observation of a small percentage (1-5% of MNC) of NK in the ECA(+) population. This technique was found to be advantageous for the study of NK heterogeneity and NK biology.