- The role of phospholipase A 2 (PLA 2 ) and its metabolite arachidonic acid (AA) in the proliferation and differentiation of HL-60 cells was investigated. Addition of either 1,25-dihydroxyvitamin D 3 (1,25(OH) 2 D 3 ) or retinoic acid (RA) to HL-60 cells for 2 h inhibited PMA-stimulated PLA 2 activity measured by [ 3 H]AA release. The inhibitor of PLA 2 activity, p -bromophenacyl bromide (BPB), significantly inhibited the proliferation of HL-60 cells and of fibroblast L929 and Swiss 3T3 cells in a dose-dependent manner. The effect of BPB on proliferation is probably through its inhibitory effect on PLA 2 activity, since the same doses of BPB which inhibited proliferation also inhibited PLA 2 activity determined by [ 3 H]AA release. The importance of PLA 2 activity for cell growth was further supported by the effect of two other PLA 2 inhibitors, AACOCF 3 and scalaradial, which inhibited HL-60 proliferation in a dose-dependent manner. BPB, AACOCF 3 and scalaradial significantly increased the doubling time to 32.4 h, 34.0 h and 31.8 h, respectively, compared with 24.6 h in the control. The inhibitory effect of BPB on HL-60 proliferation was reversed by addition of exogenous free AA to HL-60 cells, indicating the importance of this metabolite for the proliferation process. This reversible effect is specific for AA since it was not achieved by other fatty acids like linolenic acid (LA) or oleic acid (OA). Addition of free AA to HL-60 cells did not induce differentiation, as expected. Although BPB, AACOCF 3 , or scalaradial inhibited proliferation, they did not induce differentiation nor affect the differentiation induced by 1,25(OH) 2 D 3 or RA. These results implicate that PLA 2 activity has no regulatory role in differentiation of HL-60 cells. The differential effect of PLA 2 inhibitors on proliferation and differentiation of HL-60 cells suggests that these two processes function under different regulatory mechanisms.