The effect of harmful compounds such as KCN, phenol and herbicides such as 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 1-chloro-3-ethylamino-5-isopropylamino-2,4,6-triazine (atrazine) and 2-N-tert-butyl-4-N-ethyl-6-methylsulfanyl-1,3,5-triazine-2,4-diamine (terbtryne) on the photocurrent of photosynthetic membranes (so-called chromatophores) was investigated by using the carbon paste electrode. The amperometric curve (photocurrent-time curve) due to the photo-induced electron transfer from chromatophores of the purple photosynthetic bacterium Rhodobacter sphaeroides to the electrode via an exogenous electron acceptor was composed of characteristic two phases: the abrupt increased current immediately after illumination (I₀), and the constant current with time (I_c). 2,5-Dichloro-1,4-benzoquinone (DCBQ) was found to be most useful exogenous electron acceptor for this system than other redox compounds. Photo-reduction of DCBQ was observed as a Michaelis-Menten type kinetics, and reduction rates were dependent on the amount of DCBQ and photon flux intensity. In the presence of KCN, the only I_c decreased depending on the concentration of KCN higher than 0.05 μM (= μmol dm^-3). The I₀ decreased selectively by addition of phenol at the concentration higher than 20 μM. Terbtryne also affected only I_c when the concentration of terbtryne was higher than 10 μM. On the other hand, DCMU and atrazine did not affect both I₀ and I_c. The utility of this electrode system as the detection of harmful compounds will be discussed in the conference.