The NB4 cell line, established from a patient with APL, carries the t(15; 17) and undergoes differentiation along the granulocytic pathway when exposed to retinoic acid (RA). The NB4 cell line was used as a model for exploring the expression of genes and proteins implicated in growth regulation, differentiation and apoptosis during treatment with RA. NB4 cells undergo a series of cytological and molecular alterations during RA treatment--Day 1: cell differentiation marked by an increase in CD11b is evident. Day 2: WAF1/p21 mRNA and then protein rise, though they drop 2 days later. Day 3: the percentage of cells in S phase begins to decrease and G1 arrest begins. Day 4: p53 mRNA level and then protein levels fall. Day 5: CD11b/BCL-2 double staining cells are markedly reduced. No signs of apoptosis were observed after up to 8 days of treatment with RA. These results demonstrate that NB4 cells treated with RA rapidly differentiate and arrest at G1 phase concurrent with p53-independent WAF1/p21 induction; in addition, phenotypic differentiation appears to commence before changes in cell cycle progression. An explanation for the decrease in p53 as well as the lack of apoptosis immediately after BCL-2 downregulation will require further study.