Mouse MAb to the early protein, p41/38 of HHV-6A originating from ascites fluid and purified by protein G affinity chromatography. This MAb is used in conjunction with HHV-6 A/B MAb to early phosphoprotein p41, can differentially identify HHV-6 variant A or variant B isolates and can be used to detect early viral protein expression in infected cells, indicating active virus replication. This MAb can be used in immunofluorescence and/or immunohistochemistry detection system. Does not cross react with HHV-6B.