Pigment analysis was performed by means of normal phase HPLC on a number of bacteriochlorophyll a and b containing species of purple bacteria that contain a core antenna only. At least 99% of the bacteriochlorophyll in Rhodobacter sphaeroides R26, Rhodopseudomonas viridis and Thiocapsa pfennigii was esterified with phytol (BChl a p and BChl b p, respectively). Rhodospirillum rubrum contained only BChl a esterified with geranyl-geraniol (BChl a GG). Rhodospirillum sodomense and Rhodopseudomonas marina contained, in addition to BChl a p, small amounts of BChl a GG, and presumably also of BChl a esterified with dihydro and tetrahydro geranyl-geraniol (Δ2,10,14-phytatrienol and probably Δ2,14-phytadienol). In all species bacteriopheophytin (BPhe) esterified with phytol was present. The BChl/BPhe ratio indicated that in these species a constant number of 25 ± 3 antenna BChls is present per reaction centre. This number supports a model in which the core antenna consists of 12 α-β heterodimers surrounding the reaction centre. Determination of the in vivo extinction coefficient of BChl in the core-reaction centre complex yielded a value of ca. 140 mM(-1) cm(-1) for BChl a containing species and of 130 mM(-1) cm(-1) for Rhodopseudomonas viridis.