Full-length cDNA species encoding two forms of acyl-CoA synthetase from a K-562 human erythroleukaemic cell line were cloned, sequenced and expressed. The first form, named long-chain acyl-CoA synthetase 5 (LACS5), was found to be a novel, unreported, human acyl-CoA synthetase with high similarity to rat brain ACS2 (91% identical). The second form (66% identical with LACS5) was 97% identical with human liver LACS1. The LACS5 gene encodes a highly expressed 2.9 kb mRNA transcript in human haemopoietic stem cells from cord blood, bone marrow, reticulocytes and fetal blood cells derived from fetal liver. An additional 6.3 kb transcript is also found in these erythrocyte precursors; 2.9 and 9.6 kb transcripts of LACS5 are found in human brain, but transcripts are virtually absent from human heart, kidney, liver, lung, pancreas, spleen and skeletal muscle. The 78 kDa expressed LACS5 protein used the long-chain fatty acids palmitic acid, oleic acid and arachidonic acid as substrates. Antibodies directed against LACS5 cross-reacted with erythrocyte membranes. We conclude that early erythrocyte precursors express at least two different forms of acyl-CoA synthetase and that LACS5 is present in mature erythrocyte plasma membranes.