Circulating lymphocytes were enumerated in 28 patients with Crohn's disease and in 12 patients with other diseases by rosetting and by immunofluorescent staining using monoclonal antibodies for T-cell surface phenotypic markers [OKT3 (mature), OKT4 (helper), and OKT8 (suppressor/cytotoxic)] or polyvalent antisera for surface immunoglobulins (B cells). Total lymphocyte counts were reduced only in those with non-steroid-treated active Crohn's disease. Circulating monocyte counts, proportions of peripheral T and B cells, and percentages and absolute numbers of mature, helper, and suppressor T-cell subclasses in Crohn's disease were not significantly different than in the controls. Helper to suppressor T-cell ratios were comparable in all subjects, varying directly with numbers of helper T cells (p less than 0.05). Individual ratios of helper to suppressor T cells did not correlate with disease activity or location, the use of steroids, serum albumin, or total lymphocyte or monocyte counts. This study provides no evidence for underlying abnormalities of circulating lymphocyte subpopulations in Crohn's disease when compared to subjects with other illnesses. The characterization of lymphocyte subclasses in affected tissues is an important area of continuing investigation.