Changes in the light transmission of suspensions of activated neutrophils are widely used to measure the dynamics of neutrophil aggregation. Such studies have suggested, for example, that aggregation is irreversible for human newborn neutrophils but fully reversible for adult cells. We have evaluated aggregation directly by microscopic particle counting and compared it with changes in light transmission (delta T) and with release from three granule subsets for neutrophils activated with N-formyl-methionyl-leucyl-phenylalanine (FMLP). Maximal increases in %T in response to 0.5 micromol/L FMLP were approximately 25% larger for newborn than for adult neutrophils, and were only partially reversible by 8 minutes, while %T increases for adult neutrophils were fully reversible. However, measurements of neutrophil aggregation using light microscopy showed that both newborn and adult neutrophils fully deaggregated. A further independence of delta T from aggregation was found by pretreating adult neutrophils with cytochalasin B (5 micrograms/mL) in the presence of 0.5% gelatin, a pretreatment that blocked FMLP-induced neutrophil aggregation while allowing large increases in %T and degranulation. In response to FMLP, newborn neutrophils released more enzyme from each granule subset than did adult neutrophils. Our results suggest that cellular events associated with neutrophil activation (other than aggregation) are implicated in light transmission responses and that these differ for adults and newborns. These results also suggest that reports of neutrophil aggregation should be based on direct particle counting methods rather than on %T responses.