Studies with fluorescent Ca2+ indicators in large populations of neutrophils in suspension reveal a stable base line followed by a rapid agonist-induced elevation of cytosolic free calcium, [Ca2+]i, concomitant with other parameters of cellular activation. To study the role of adhesion in cell activation, we monitored [Ca2+]i in single neutrophils adhered to albumin-coated or fibronectin-coated glass coverslips before and after stimulation with the chemotactic peptide N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP). Human neutrophils loaded with 2 microM fura 2/AM were allowed to adhere to coverslips for 15-20 min at 37 degrees C. [Ca2+]i was monitored with a dual excitation microfluorimeter with a time resolution of 200 ms. Statistical analysis was performed using an algorithm allowing to detect significant [Ca2+]i peaks. 54% of the cells showed spontaneous [Ca2+]i oscillations. The amplitude of these [Ca2+]i peaks averaged 77 +/- 10 nM above basal levels (mean value of 110 +/- 20 nM), and their mean duration was 28 +/- 5 s; periods of [Ca2+]i bursts could last up to 15 min. In "silent" cells exhibiting a stable [Ca2+]i base line without spontaneous oscillations, low concentrations of fMLP (10(-10)-10(-9) M) could induce sustained [Ca2+]i oscillations. By contrast, higher agonist concentrations (10(-6) M) induced a single [Ca2+]i transient followed by a stable base line. 47% of the cells showing spontaneous [Ca2+]i oscillations did not respond to fMLP. Spontaneous [Ca2+]i oscillations depended on the continuous presence of extracellular Ca2+. Therefore: (i) spontaneous oscillations of [Ca2+]i occur in neutrophils adherent to various substrata; (ii) these oscillations do not preclude and can be dissociated from the response to fMLP; (iii) neutrophil functions might be controlled by [Ca2+]i oscillations rather than by sustained alterations of [Ca2+]i.