We have investigated the impaired secretion response of neonatal platelets. We compared the response of washed neonatal and adult platelets to thrombin and collagen, and to specific activators of calcium flux (inositol trisphosphate) and protein kinase C activation (oleoylacetyl glycerol). Neonatal platelets show no impairment of aggregation, secretion of [14C]serotonin or phosphorylation of specific intracellular proteins in response to thrombin, inositol trisphosphate, or oleoyl-acetyl glycerol. However, neonatal platelets have a markedly decreased response to collagen. To further evaluate this deficient response, we examined specific aspects of the collagen activation pathway. Collagen-platelet interaction as measured by adhesion of platelets to collagen-coated dishes showed no difference in adhesion of neonatal platelets compared to adult controls (20.1 ±11.6versus18.6 ± 9.3%). The presence of GPIa/ Ila, a Mg2+-dependent collagen receptor, was evaluated by flow cytometric analysis of binding of fluorescein-tagged monoclonal antibody, 6F1 (directed against GPIa/IIa). There was no difference either in the percent of platelets that bound antibody (80versus81%) or in the mean fluorescence intensity of the adult and neonatal samples. Phosphoinositide hydrolysis was decreased in neonatal platelets in response to collagen but normal in response to thrombin. Neonatal platelets released more arachidonic acid than adult platelets in response to thrombin (29.5 ± 3.2versus19.6 ± 1.8%) but less than adult platelets in response to 10 µg/mL collagen (3.2 ± 1.1versus9.3 ± 3.0%). Thromboxane B2production was also decreased in response to collagen (52.7 ± 12.6versus101.4 ± 18.7 ng/mL). These results suggest that the deficient collagen response in neonatal platelets may lie in transduction of the collagen signal to phospholipases A2and C.