Eight symptomatic individuals chronically exposed to indoor formaldehyde (HCHO) at low concentrations (0.07-0.55 ppm) were compared to 8 nonexposed subjects with respect to: (1) presence of IgG and IgE antibodies to HCHO conjugated to human serum albumin (F-HSA); (2) the percentage of venous blood T and B cells by E and EAC-rosetting; and (3) the ability of T and B cells to undergo mitogen (PHA, PWM) stimulated blastogenesis as measured by the incorporation of tritiated thymidine. Anti-F-HSA IgG, but no IgE, antibodies were detected in the sera of the 8 exposed subjects; none were found in 7 of the unexposed controls. T lymphocytes were decreased in the exposed (48 +/- 11.5%) compared to the control (65.9 +/- 4.97%) subjects (p greater than .001 less than .01). B cells were 12.6 +/- 1.6% (HCHO group) and 14.75 +/- 2.1% (controls) (p greater than .02 less than .05). The incorporation of labeled thymidine by T cells (PHA) was decreased: 17,882 +/- 2,293 cpm (HCHO group) and 28,576 +/- 3,807 cpm (p greater than .001 less than .01). T and B cell blastogenesis (PWM) was 9,698 +/- 1,441 cpm (HCHO group) and 11,279 +/- 1,711 (controls) (p greater than .05 less than .1). Exposure to HCHO appears to stimulate IgG antibodies to F-HSA and decrease the proportion of peripheral T cells.