The immune system of 12 healthy chronic marijuana-smoking adults was evaluated while they smoked marijuana daily for 64 consecutive days under controlled hospitalized conditions. Studies included enumeration of B and T cell subpopulations, lymphocyte proliferative responses to PHA and to allogeneic cells, and serum immunoglobulin levels. Percent B cells, initially low in 2 patients, became normal. Baseline total B cells, determined either by surface immunoglobulins (338 cells/mm3 +/- 60 SEM) or complement receptors (162 cells/mms +/- 27) were significantly (p less than 0.05) less than control but increased to normal (485 +/- 97 and 239 +/- 47) over time. Percent T cells, initially low (less than 40%) in 4 patients, became normal. Baseline T cells (951 cells/mm3 +/- 70 SEM), significantly lower than controls (2,010 +/- 210, p less than 0.05), increased to normal by day 63 (1,875 +/- 281). In vitro lymphocyte response to graded doses of PHA and to allogeneic cells was normal initially and did not change over time. Serum IgG (1,064 +/- 33), IgA (166 +/- 13), and IgM (96 +/- 6) were normal. Serum IgE levels increased in 4 subjects without evidence of allergy. Short-term chronic marijuana use does not have a substantial adverse effect on B or T cells of young healthy adults.