HLA class II antigens were studied in a panel of 130 Nigerians. Complex patterns of associations were seen between HLA-Dw, -DR and -DQ specificities, differing widely from those reported for other populations. A number of Dw types were associated with the same DR antigen: Dw'1N' and Dw'BERN' with DR1, Dw2 and Dw'2N' with DR2, Dw5 and Dw'5N' (Dw5 + Dw'F5') with DRw11. It was also observed that a Dw type associated with more than one DR antigen: HLA-Dw3 was assigned to individuals who were DR3 negative and similarly Dw10, Dw13 and Dw14 to individuals negative for DR4. HLA-DRw8 and Dw8 were completely dissociated in Nigerians, and Dw8 did not show a preferential DR association. These results demonstrate that DR and DQ identity between HTC stimulator and responder cell is not necessarily a prerequisite for Dw to be assigned. Preliminary studies show that subtypes of HLA-Dw1 and Dw8 detected by HTC typing correlate with restriction fragment length polymorphisms (RFLPs) detected with a combination of Bgl II enzyme and DRA/DRB cDNA probes. HLA-DP antigen frequencies differed between Nigerians and British Caucasoids. The most common DP antigen in Nigerians was DPw1, compared with DPw4 in Caucasoids. HLA-DPw6 appeared to be absent or rare in both Nigerians and British Caucasoids. Only five out of 68 Nigerians tested were assigned two DP specificities. The association between HLA-DR3 and DPw1 reported in Caucasoid panels was absent in Nigerians.