['Acute myeloid leukemia (AML) cells express the cell surface antigen CD33 that can function as a downregulator of cell growth, mediating growth arrest and apoptosis. The protein kinase Syk is an essential element in several cascades coupling certain antigen receptors to cell responses. Recently we reported that CD33 recruits Syk for its signaling in AML cell lines. In this study, we further investigated the mechanism(s) of Syk engagement in CD33 signaling in primary AML samples.', 'We investigated 25 primary AML samples for their proliferative response (3H-thymidine incorporation) and biochemical changes (Western blot analysis) to anti-CD33 mAb treatment.', 'Proliferation studies demonstrated that 14 (56%) of AML samples were responsive (R) while 11 (44%) were nonresponsive (n-R) to inhibitory antibody activity. Seven of 25 AML samples (28%) expressed undetectable levels of Syk. However, cells from two of these patients expressed the ZAP-70 protein kinase. In Syk/ZAP-70(+) samples, CD33 ligation inhibited proliferation in 70% of cases, while none of the Syk/ZAP-70(-) samples was responsive. There were significant biochemical differences between responder and nonresponder AML populations. In responder samples, CD33 ligation induced phosphorylation of CD33 andSyk and formation of the CD33/Syk complex. In nonresponder samples, CD33 was not phosphorylated, and Syk was in complex with the SHP-1 protein phosphatase constitutively.'] Does the inhibitory effect of anti-CD33 monoclonal antibodies on AML cell growth correlate with Syk and/or ZAP-70 expression?