Leucocytes from 14 cases of neurocysticercosis who had multiple lesions showed a significantly lower level of chemotaxis than those from healthy controls [with mean (S.D.) movements of 49.86 (15.0) and 89.1 (12.9) microns, respectively; P < 0.05). In contrast, the leucocytes from 14 cases with single lesions had similar chemotactic responses to the controls. Although the level of lymphoproliferation stimulated with phytohaemagglutinin, a measure of T-lymphocyte function, was also considerably less using cells from the multiple-lesion cases than when cells from the single-lesion cases were used, the difference was not statistically significant [0.644 (0.248) of control values v. 0.463 (0.317); P < 0.1]. Flow cytometry indicated that the total numbers of CD4 cells and CD4/CD8 ratios in all blood samples were normal. It therefore appears that, among neurocysticercosis patients, only those with multiple lesions have defective neutrophils and may also have dysfunctional lymphocytes.