Luminol-enhanced chemiluminescence assay was used to detect the surface expression and the consequent activation of receptors (FcRI and FcRII) of murine macrophages (M phi s). When murine IgG2a was used for the specific detection of FcRI and IgG2b for FcRII, a newly established procedure enabled us to detect the activation of each receptor with as few as 3 X 10(5) M phi s. Briefly, TNP-SRBC coated with monoclonal IgG2a or IgG2b antibodies directed to TNP (sensitized SRBC) were used as reagent, in the presence of 1 X 10(-5) M luminol, and the emission was measured with a liquid scintillation counter. When results obtained by chemiluminescence counting were compared to the results obtained by the rosette formation by adding the same SRBC reagent to peritoneal M phi s obtained after ip injection of Listeria, fortified chemiluminescence counting allowed us to obtain a more definite answer about the activation of each receptor. Under the conditions established, the specific activation of FcRI was obtained by the addition of rIFN alpha A/D to the resident M phi s in vitro and the specific activation of spleen M phi FcRII by iv injection of IAP (Immunosuppressive acidic protein) into mice. These two results supported the independence of the two receptors detected by the assay.