The present study concerns a panel of 33 acute non lymphoblastic leukaemia (ANLL) patients, previously typed for HLA-A,B serological specificities and including samples with a normal HLA-A,B phenotype (3,4 detected specificities) as well as samples with missing and extra specificities. Samples were analysed at the protein and/or RNA level in order to verify whether the observed typing anomalies were due to a modified quantitative expression of class I molecules. The number of HLA-A,B assigned specificities correlated significantly with the cell surface class I expression detected by indirect immunofluorescence using the monomorphic anti-class I MoAb W6/32 (Spearman rank correlation test, P < 0.01) and with the amount of class I Heavy Chain (HC, P < 0.05) and beta-2-microglobulin (beta 2m, P < 0.05) evaluated by Western blot in whole cell extracts. The RNA analysis suggested a HC-beta 2m coordinated down regulation at the mRNA level in a patient with no assigned HLA-A,B specificities. Another patient with no detectable HLA-A,B specificities showed a low expression selectively of the beta 2m protein. The results reported here demonstrate a heterogenous quantitative HLA class I expression in ANLL blasts, analogous to results reported for solid tumours.